How to Find the Defective Battery in Your Battery Bank

September 24, 2013 · 26 comments

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IUsing a good quality voltmeter to diagnose your battteriest only takes one bad battery to bring your off grid power system to a complete halt. Even in systems that have 8, 16, 24 or even more batteries, one defective battery can drain every good battery and leave you powerless. Unfortunately it is not always easy to find the battery causing all the trouble. However, if you follow the steps in this article, you will be able to pick out the bad apple within an hour or so.

The story is usually the same. You have been living off the grid for awhile and although you don’t have unlimited power, you are learning to live with what you got. But lately things aren’t working as well. You don’t think you are using more power than usual and you think your solar modules, wind turbine, water turbine or generator are working normally. But it seems like the batteries are always low, or the voltage drops off very fast even when the batteries should be full. Time to check your battery bank.

Some of the signs you might have a defective battery.

Poor Performance – The most common symptom of a bad battery is that your power system is not working as well as it used to. After months of operating your power system you will get good at predicting battery voltage and how much capacity you have. When a battery starts to cause problems, the voltage will be lower than usual or your bank will have less capacity than it should. Sometimes this decline in performance is very gradual (more difficult to diagnose) and sometimes it is immediate.

Excess Heat – When opening your battery box it is common for a buildup of heat when a battery is failing. The temperature inside your box is another factor you will get good at estimating. Most of the time your batteries will remain cool but in times of heavy charging or heavy discharging they will make more heat. A defective battery will sometimes make heat all the time. If a cell is shorted inside the battery it will make heat 24 hours a day and discharge your other batteries producing even more heat.

Requires More Water – Another common sign of a bad battery is the requirement to add more distilled water than usual. Sometimes only the defective battery requires more water and sometimes the whole bank will require more water than usual.



Before disconnecting anything, take a look inside your battery bank.

Do you see:

  • any leaking batteries? Any electrolyte in the bottom of your battery box?
  • excess moisture on top of any battery?
  • excess dirt on top of any battery?
  • excess corrosion on any battery?
  • any signs of heat on any battery?
  • are any of the batteries swelled up or expanded?

Defective batteries usually gas more than good batteries forming extra dirt, electrolyte or corrosion on top of the battery. If you see one battery with any of the above signs, you have probably found the bad battery already. If all of the batteries seem the same, proceed to the next step.


Next feel each battery. Are any of them hotter than the others? The hot one will likely be the damaged battery. When a battery is shorted inside it might make heat all the time. It is also possible for a shorted battery to only make heat intermittently. If one of your batteries is much warmer than the others, it is a definite sign something is wrong.


Remove the caps. Are any of the battery’s electrolyte lower than the others? Sometimes there will only be one faulty cell. You must look inside all the cells of every battery. Any battery that is low enough that the plates are exposed is a problem. If one battery is only down a little and the plates are all covered, it is possible the battery is beginning to fail. If all the batteries (or the majority) are down, it is simply time to add distilled water. Learn about adding water and good battery maintenance tips here.

If you find a battery that has one or more low cells (compared to all the others), you have likely found the bad battery. If a battery is low, take a little extra time to look for a crack in the housing (container) of the low battery. Sometimes there is a tiny crack right at the level of the electrolyte. It might not be leaking anymore as the electrolyte is now below the crack.


If all the batteries have plenty of electrolyte, get a good flashlight and look down into the cell of each battery.

  • Do the plates looks clean?
  • Are any of the plates swelled up or damaged? (As in photo below)
  • Do any of the plates have more sulfation (white coating) than the others?
  • Is it black inside (more than the others), or does it look burnt more than the others?
  • Is the electrolyte clear or really dirty?

It is important to note that batteries won’t always look like new inside. There will be dirt and the electrolyte will look burnt over time. What you are really looking for is inconsistency between batteries or cells. If all cells of all batteries are dirty, then all is well. But if only one or two batteries are dirty inside, you have likely found the problem(s).

12 volt battery that is damaged.

A high quality hydrometer is essential to diagnose your batteries.STEP 5:

Check the specific gravity (S.G.) of the electrolyte of each cell of each battery. Get a good quality, temperature compensated battery hydrometer for this test. Like the previous tests you are looking for inconsistency. The S.G. will vary constantly depending on the depth of discharge of your batteries, the amount of electrolyte in each cell, how long it has been since your last equalization charge and how long it has been since you added distilled water.

If you have a problem battery, the specific gravity will be much different than the others. Usually the defective battery will have an unusually low S.G. but an unusually high reading may indicate a problem as well. Even one cell with an unusually low S.G. is enough evidence to indicate a problem battery.


With all the batteries still connected by the cables, check the voltage of each battery while they are at rest. Any significant differences?

While applying a charge to the battery bank, check the voltage of each individual battery. A faulty battery will usually show a higher voltage than the others while being charged. The higher voltage is a result of the battery not having as much capacity as the others.

While discharging the battery bank, check the individual voltage of each battery. A troubled battery will show a lower voltage than the other batteries. This lower voltage is also due to the reduced capacity.

If you get a strange voltage reading during charging or discharging, it is very important the you take a good look at the surrounding battery cables and lugs.

  • Are they tight?
  • Do they feel hot or look like they have been hot or burnt?
  • Are the cable lugs simply crimped or soldered as well?

It is VERY COMMON to have a defective battery cable/cable lug that will act as if the battery itself is the problem. If the lug was only crimped, many years of being heated and cooled can cause the lug to become loose. A loose cable/lug will decrease current flow to the surrounding batteries and make the battery appear to be defective. A loose cable lug can also make a lot of heat, so much heat it can melt the cable or the terminal on top of the battery. A loose connection can act like an electric “stick welder” and either fuse itself together or melt everything.

Golf cart 6 battery with a melted post.

In the above photo, the crimped on lug on the interconnect cable was loose. The loose lug got hotter and hotter until it was hot enough to melt the lead battery post. Strangely enough you will see a nice, brand new cable attached to the melted stud/post. This was an attempt to fool the battery supplier into warrantying the golf cart battery for a defective post. Any experienced battery installer/supplier would not be tricked by this. You can see the nice heat shrink (which would have melted off), a clean looking lug (which would normally be discolored from the heat) and the melted indent on the top of the battery from the faulty cable.


If you have not found the problem battery yet (which would be very unusual), it is now time to remove all the battery cables and interconnect cables.

Before removing the cables, do your best to charge the battery bank as much as possible. If the battery bank has been sitting around for awhile without being charged it is possible the bad battery will lower the voltages and specific gravities of the good batteries.


With all cables removed, check the voltage of each battery. A battery with a shorted cell will have its voltage drop very fast when disconnected from the rest of the bank. When connected to the rest of the batteries, they will support the bad battery and hold the voltage as long as possible. Once the battery is isolated it cannot hold its voltage for more than a few minutes.

If all the voltages are stable, then it is unlikely you have a shorted cell. It doesn’t mean your batteries are all good. It just means your battery is not short circuited inside.


Check the specific gravities of all of the cells of all of the batteries as in STEP 5. Any wild variation will tell you there is a faulty battery/cell. The faulty battery will likely have a very low S.G. when removed from the rest of the pack.

STEP 10:

Now you will need to remove your batteries from the box so you can have a good look at each of them. Does the case (plastic container) look like it is swelled or expanded? It will look like someone hooked it up to an air compressor and over filled it. Any battery that is swollen has little, if any, life left.

Repeat STEP 1 and STEP 3 now that the batteries are out of the box and much easier to look at.

If all of this work has left you frustrated by the fact you haven’t found any problems yet, be patient, you have not tried everything yet.

STEP 11:

The last step is to load test each individual battery until you find the defective unit. You can obviously take your batteries to a professional battery installer/supplier and have them perform the tests but that is a lot of heavy work.

Instead you could do some crude load testing yourself.


Using a 12 volt bulb as a crude load tester.If your batteries are 12 volt units it is pretty easy to find 12 volt loads such as light bulbs or heaters. RV and automotive stores have lots of 12 volt light bulbs.

You should be able to find 25, 50, 100 and 200 watt 12 volt bulbs for your tests.

Each 100 watts of 12 volt light bulbs should remove about 8 amps from your 12 volt battery.

Using the manufacturer’s literature find the C5 rating for your battery and divide by 5. This is the amperage of our load testing load.

For example:

Your battery is C5 rated 100 amp hours at 12 volts.


100 AH / 5 = 20 AMPS (at 12 volts) = 240 WATTS

For this test we would need two 100 watt, 12 volt light bulbs and one 50 watt, 12 volt light bulb.

Now test each battery by applying the load (light bulbs) and monitoring the battery voltage. If you have unlimited time you could keep the load connected until the battery reaches 10.5 volts just like the manufacturers. However you could also keep the load connected for 30, 60 or 90 minutes, record the battery voltage, and compare to the other batteries in your bank. The faulty battery will see a much more drastic voltage drop then the others.


Load testing 6 volt batteries is a little more complicated as 6 volt loads aren’t that readily available.

You have two options:

  1. Test two batteries at a time in series and test just like a 12 volt battery
  2. Make 6 volt loads from 12 volt loads

Using two batteries in series is doable but it is a hassle. First you would test all pairs of batteries as above. One or two pairs will likely be under-performing. If you have a pair that under-performs, you will  now have to test each of those batteries. The problem is they will probably be dead and have to be charged, as well as, two more batteries to pair with each of the potentially defective batteries. It can be frustrating. We prefer making 6 volt loads from 12 volt loads.

As stated above, 6 volt loads are not easy to find. However you can use 12 volt loads and derate them to 6 volts.

Any load designed for 12 volts will use 1/4 the rated power (wattage) at 6 volts.

Why 1/4 and not 1/2 the power? Read this article about using 120 or 240 volt heaters as dumps loads as it explains how a resistor/heater consumes power at different voltages.

This time we will use a Trojan T105 as our battery of choice. The C5 AH rating for a T105 is 185 amp hours at 6 volts.


185 AH / 5 = 37 AMPS = 222 WATTS

As we are using 12 volt bulbs we need to multiply 222 watts by 4 to make up for the de-rating of the 12 volt bulbs at 6 volts.

Bulbs required  = 888 watts at 12 volts

For this test we would need 8 100 watt,  1 50 watt, and 1 25 watt 12 volt light bulbs. If you don’t want to mess around with light bulbs, buy a resistor, 12 volt hair dryer or 12 volt car heater.

Connect your new load to each battery for a set amount of time. It could be 30, 60, 90 minutes or even a few hours.

Monitor the voltage of each battery and discard the battery or batteries that do not hold their voltage as well as the others.

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{ 26 comments… read them below or add one }

Fred October 12, 2016 at 5:08 am

I have 2 battery system on the diesel generator that has its own charging system. both batteries , cell levels were good , 3 weeks later generator didn’t start. Found 1 battery to be almost out of water. What would cause this problem? didn’t notice any extra corrosion


Jody Graham October 13, 2016 at 8:45 am

Hi Fred,

Thank you so much for your question.

If three weeks ago your batteries were full of water and operating normally but now one battery is almost dry of water, I would suspect a small crack somewhere in the case of the battery.

If your generator was overcharging the starting batteries, both of the batteries should be low on water.

Look for dampness underneath the battery that is losing the water.

If none of this has been helpful, please give me some more details:

Brand of batteries
Age of batteries
Model number of batteries
Make and model of generator
How low is the low battery?
Is it just below the plates or is it down three quarters?


J.R. October 8, 2016 at 1:08 pm

Hi all, In my first post, I failed to mention I think this 36v cart has been sitting unused for a couple of years, and no one charged the batteries. The battery date codes are “B-0”, so I am assuming this is Feb, 2010. the two batteries in the center of the cart were the longest getting to turn on. If I check the voltage from the pos terminal of the first battery with the neg terminal of the last battery and this is less than 36v, will that mean the cart will not attempt to move ? I just don’t know at what point I need to carry this cart to a repair garage. Thx!


Jody Graham October 11, 2016 at 9:48 am

Hi J.R.,

Thanks for your question. If the cart has been sitting unused for two years it is likely time for a new battery bank. I presume the battery voltage was at or near zero when you got the the golf cart. I’m not sure what you mean by the middle batteries being the longest to turn on. If you mean charge up, that is very typical. The batteries closest to the battery cables tend to charge first and work their way in. The bad part of that is the batteries closet to the cables take the most abuse and tend to wear before the others. Every golf cart has a lower voltage limit at which the cart will not work. A full 36v battery bank should show a voltage of at least 37.5VDC and I wouldn’t expect your golf cart to shut down before 35.4 VDC or even lower. If your battery voltage is in that range then a trip to the shop is likely going to have to happen. If you put a charger on the batteries and get them above 36VDC with the charger still connected will the cart move? Jody


J.R. October 8, 2016 at 1:00 pm

Hello all, I recently purchased a used 1992 Club Car 36v Golf cart. The “B-0” batteries needed water, so that was the first action I took to attempt to see if it would run. I did notice a faint alarm if I shifted into reverse. I used a 12v charger to create a charge on two batteries at a time, then my Club Car charged started to charge the entire battery bank. when it charges until the automatic charger cuts off, I turn on the key, put it into forward, and press the go pedal, but all I get is a click and a faint hum…no movement. My question is, should it be at least trying to move, or does it have to have at least 6 fairly good batteries in order for the cart to act like it moves ? I hoped that if 5 of the batteries were OK and one was bad, that the cart would move but only slowly, and would die easily. Does anyone know if all six batteries must be of OK condition for the cart to begin to operate, or will it just “sit there” ? Thx!


Jody Graham October 11, 2016 at 9:59 am

Hi J.R.,

As I already replied to your second message I’ll just give you my thoughts for this one quickly. If your cart is charging the batteries up to close or above the 37 volt range and the batteries are holding that voltage I would suspect the solenoid (which is just a massive mechanical relay like the one used to operate the starter on your vehicle) is corroded and either needs to be taken apart and the corrosion removed (if you are the mechanical type) or replaced. The fact that you are getting the click tells me the relay is closing to try and make the connection but corrosion is stopping the connection. This is very common with higher voltage DC as they tend to arc every time the the relay opens and closes. Just the reality of DC. I would start at the main solenoid which is controlled by the gas pedal. Try to find the click and look for the solenoid.



Camille September 3, 2016 at 3:26 pm

Hello! I have a battery bank of 4 very old 6V batteries wired series-parallel. My electrical system has completely stopped working,
And even after running my generator for a couple days, the batteries are not charging. However, I really don’t think it’s the charge controller. I know I need to replace the batteries but I’m closing up my cabin for winter and am not sure if it’s safe to leave the solar panel and charge controller set up without batteries attached. What do you think?

I would like to take out the 2 worst and see if I can leave 2 connected. How do I do this safely?

Thanks! I have spent hours googling this, reading posts, and watching videos, but can’t really find an answer.


Jody Graham September 5, 2016 at 4:33 pm

Hi Camille,

Thank you for your question.
You can most definitely take the batteries from your system without harming your solar modules or charge controller. Nothing will be harmed. Just remove the batteries. If there is a breaker between the solar panels and the controller you could turn that breaker off if you wish. But it is not necessary. Turning the breaker off will remove any voltage potential from the system making it impossible for someone to short the wires.

I will assume you have used the battery troubleshooting info in this article.

To pick the best two batteries in your system just disconnect all the battery cables and let the batteries sit for a few hours. Measure the voltage of each battery. Pick the batteries that have the highest voltage. Those will be your best batteries to keep connected if you still prefer to keep the controller connected to batteries. Thanks…Jody


Jorge May 15, 2016 at 1:10 am

Hello, I recently recovered my grandfather’s 1997 Yamaha G16e golf cart. It had been sitting at my local golf course since he passed away 6 years ago. The batteries were like 1-2 years old and he hardly played during that time as he was ill. I just towed it to my house, and started cleaning it. There is no battery charger available, so before I start putting in too much money into it I wanted to see if it worked. Here are a couple of questions:
1. Can I charge these batteries with a regular car battery charger? Individually or in pairs of 2 of course.
2. I tested the batteries with a battery load tester, and 5 of them gave a reading of 2 volts while one of them gave around 1 volt rating. This reading was done while the batteries were dry. I haven’t purchased any distilled water yet, but that’s my next step. Is this a clear indicator that they are dead? Any chance of getting them back to life just enough to see if the motor is fine?


Jody Graham May 16, 2016 at 9:20 pm

Hi Jorge,
Sorry to hear about your grandfather. Six years is a long time for a golf cart battery to sit dormant. Yes you can charge with a normal car charger as long as the battery voltage of the golf cart is 12 volts and your charger is 12 volt. If the battery charger is adjustable for amps you will likely want to set it to charge at least at 20 amps or so. The smaller the charger the longer it will take to charge them. I guess I am curious why the batteries are dry? DO NOT charge them at all until you have at least covered all the lead plates on each battery. You should be able to use a flashlight and see the lead plates. Don’t overfill as they will naturally become more full of water as you charge them.

After topping up with water you should see the voltage rise automatically and after applying a good charge you should know quickly whether the motor is good or not. If you water and charge the batteries and you get no power at the motor first make sure the lights work. If they work then have someone tap the motor gently with a hammer while you press the gas pedal as sometimes those motors can get stuck from sitting but will work fine after a few minutes of driving.

If you cannot get any of the lighting working, check the main battery cables and then the fuses. Good luck and let us know how you make out…Jody


Michael March 28, 2016 at 9:30 pm

Recently bought a 36v golf cart. All batteries reads around 6.2-6.4 except one, it reads 4.7. Do all batteries need replaced or just the one that reads 4.7?


Jody Graham March 29, 2016 at 8:00 am

Hi Michael,

Great question. Since you don’t really know the history of the battery bank and only one battery seems to be having any trouble, I would just replace the battery with the at rest voltage of 4.7 volts. Why spend money you don’t need to? It likely was a dud or ran out of water at some point. Would definitely try to replace the battery with the same manufacturer and model number if possible. Have a great day…Jody


Shirley March 4, 2016 at 2:33 pm

Hi, my 6 volt cart batteries, 2 of them have started to swell which means they are obviously faulty and need replacing. My question, is it dangerous to leave them in the cart at the moment until I get new ones, they won’t explode will they?


Jody Graham March 15, 2016 at 9:32 pm

Hi Shirley,
I am so sorry for the delay getting your question posted and replying to it. We get about 300 spam comments per day and sometimes the good ones like yours get mixed into the junk comments. Your six volt golf cart batteries are not at risk of exploding. The swelling is caused by corrosion on the plates of the batteries. The plates are packed in very tight so when corrosion forms it pushes on the outside plastic casing of the batteries. No danger of explosion really. They could short out and create a lot of heat to the point of melting or even causing a fire but that is rare…Jody


Jay February 3, 2016 at 9:16 pm

Hi Jody,
I bought a used 48v cart (4 x 12v batteries).
The cart worked great for two weeks, but now it has lost power. I checked the batteries individually. They are: 12v, 8v, 12v, 8v in sequence. The total voltage is 40v (that’s what the charger shows as well). Even with 40v, the cart should have some power. But it is not. I have not done the load test. The connectors look good.

My questions –
1. Do you think replacing the 2 batteries showing 8v should work?
2. Should I replace all batteries?
3. Could there be a engine problem?

– jay


Jody Graham February 6, 2016 at 5:28 pm

Hi Jay,
Thank you for your question.

1. My first recommendation would be to charge the two batteries (separately) that are only showing 8 volts using some kind of 12 volt charger/RV converter/automotive battery charger or solar battery charger. If each battery seems to charge but the voltage drops off quickly after removing the charge, then replace the battery.

2. Depending on the age of all the batteries it might make sense to replace all of the batteries. Obviously that would be the best option but if funds are limited you could start by just replacing the batteries that are really bad.

3. I doubt an engine problem if all was fine before. Most golf carts have a low battery disconnect to protect the batteries from damage rather than allowing the removal of too much of the battery’s capacity. Many golfer’s have been in the situation when they were driving their cart and it died all of a sudden. That is the low battery voltage disconnect. It is instant. There is usually no notice and no drop in performance. All of a sudden the voltage is too low and the cart just dies.

Hope this helps. If you get a chance to update us please do so when you get a chance…Jody


Jay February 3, 2016 at 7:32 pm

Thank you for a very nice article!
– jay


Jody Graham February 6, 2016 at 5:29 pm

You are very welcome Jay . More importantly thank you for your kind words…Jody


Ed January 13, 2016 at 6:49 pm

I have a club cart w/6 -8 volt batteries. I know 1 or more batteries must be weak. Did the following test to figure which battery is bad.
With a volt meter clamped across each battery, wrote down the starting voltage and the while driving the cart 100 yards noted the same battery voltage. The readings were:
Battery 1 Starting voltage 8.22 Under load voltage 6.8 Without load 8.05
Battery 2. 8.20 7.42 No reading
Battery 3. 9.04. 7.61. No reading
Battery 4. 8.45. 7.63. 8.22
Battery 5. 8.54. 7.80. 8.43
Battery 6. 9.07. 7.90. 8.57
Starting voltage was different for the batteries because test was at different times of day.
Do you conclude that battery # 1 , is the weakest of the bunch?


Jody Graham January 13, 2016 at 9:06 pm

Hi Ed,

I do agree with you that battery #1 is likely the weakest of the bunch. I am assuming it is also at the end of the battery bank. If so that would make a lot of sense as the outside batteries in a bank in series take most of the abuse especially if the battery cables are not that large. You sound like you know what you are doing. Good luck and let me know how things go…Jody


Ravi January 10, 2016 at 1:47 pm

Great Job, I am a student and I was curious to know how to find a faulty battery in a power bank.
thanks. 😀


Jody Graham January 13, 2016 at 9:07 pm

Thanks you for your kind words. If I can ever help you in the future please let me know…Jody


John November 17, 2015 at 8:09 pm

How can you check the Voltage of each battery while they are all connected? If they are all tied together , you will only get one reading of the entire battery bank. I can see that if you disconnect and make each battery my itself you can get separate voltage reading for each battery. Please explain . John
Reference from step 6


Jody Graham November 18, 2015 at 10:51 am

Hi John,

Thanks for your question. In a perfect world with perfect batteries you would indeed get the same voltage from each battery in a series string. You would also get the same specific gravities for each battery in the string. However as batteries age and some take more abuse than others the voltages will be different for each battery…especially if there is a problem. If you have a shorted battery for example it will have a lower voltage than the other batteries in that same string even though it is still wired in the string. The voltage in a string of batteries is just the sum of the voltages of each battery. If you have a 12.4 volt string made with two six volt batteries, one battery (from positive to negative) may show 6.0 volts and the other may measure 6.4 volts. The sum is still 12.4 volts.

Generally the batteries towards the outside of a string will sustain more abuse than the one’s in the middle of a string. When you are applying a high charge to your battery bank check the voltage on the outermost batteries and compare it with the innermost batteries and you will usually see a higher voltage on the outermost batteries. When applying a large load you will see the exact opposite. Hope this helps…Jody


Monty Vogel November 17, 2015 at 7:50 pm

Great information, nice job! I have eight 6V batteries on a solar powered camp which we just closed up, on an island. I don’t plan to go back out until Spring. The system has been running fine all Summer. I refilled the batteries a couple of weeks ago with distilled water (they take about 2 gallons each of three times/year, in the Spring, Summer, and Fall — this seems like a lot to me).
Yesterday, the Magnum 4,000 Watt inverter suddenly kept shutting off and trying to re-start. I shut it down, and shut off all circuit breakers. Turned out, the well pump, which is the only 240V circuit, was the problem. With this breaker off, or the wires disconnected at the pressure switch, the inverter worked again.
But even with a full charge, over 25..6 V, the SOC would drop rapidly whenever any load was put on, all the way down to about 22V. With the load removed, it would slowly come back up a little ways. Even with just the inverter on, and no other loads, the SOC would gradually drop, as the inverter uses a little power.
When I re-charged the 8 batteries with a generator, the SOC would quickly drop in voltage again after. And the fan in the inverter, which always comes on when charging with a generator, would no longer come on. Every load user in the camp , except for the well pump, worked fine.
So the batteries were one of the things I was suspecting after I left. Meanwhile, today I bought a new pressure switch to replace the old, which looked a little grody (this is a salt water atmosphere in Maine). I doubt that the switch or the pump is the problem, but I don’t know.
I will take all your good advice when I return in the Spring (and hopefully warmer), but I am certainly interested in your thoughts.
Thanks much in advance!


Jody Graham November 18, 2015 at 11:02 am

Hi Monty,

Thank you for your comment. Assuming you are using some type of L16 battery 2 gallons of water three times a year is high. This can by caused by having your bulk voltage too high (over 28.8 volts), equalizing too often or the bank is worn out. If you are saying your batteries at rest are 25.6 volts but quickly drop off with any load this would also indicate worn out batteries. I would assume with such high consumption there have been periods of time the lead plates have been exposed to the air. Good quality batteries will tolerate a little of this but not much. The 240 volt well pump shutting down your MS4024PAE could be an indication the batteries are worn out as well. When the pump kicks on it lowers the voltage enough that the inverter is shutting down because of LBCO low battery cut off . Let me know your thoughts, how old your batteries are, what kind they are. You could also split the two parallel banks and see if one works well while the other does not. That would point you in the right direction. Is there anything to maintain the battery charge all winter long? Thanks…Jody