Please can someone help me with this problem I'm having with my solar system? Ok I have 7 × 190w 24v panels wired in parallel, Coming down to a 80A MPPT charge controler which automatically selects whether it's a 12/24/36V and so on. From that I have 16 100A 12V battery bank wired in series, then from the batteries a 12V to 240V AC inverter which is 8000W.

Now my problem is they are not fully charging 11.8 to sometimes 12.3v it goes to only and my water pump need at least 13.5 for it to come on. Hopefully I haven't confused you guys, if u need to no any more info please ask. I'm not a electrician but I have a bit of an idea.

  • 2
    \$\begingroup\$ why does your pump need 13.5V to turn on? Is that the "On" hysterisis threshold, assuming a SLA (sealed lead-acid) battery charger circuit will take a 12V battery to 13.8V on full charge, and therefore the pump feels that it's a good time to turn on? \$\endgroup\$
    – KyranF
    Apr 8, 2014 at 0:59

3 Answers 3


I suspect that your 16 100 AH batteries are wired in parallel rather than series, otherwise you'd have ~200 volts instead of 12.

It is possible that one (or more) of your batteries has developed a shorted cell - I had this problem on my boat. The voltage would not get over 12 volts, even using a high power charger for a long time. I eventually noticed that one battery required lots of water in all but one cell - once I disconnected that battery, the other battery quickly came up to ~14 volts under charge.

If you have normal flooded cell batteries, check the water levels,looking for batteries where one cell doesn't need water, but the rest do - that (or those) batteries will be bad, and removing them from the bank should allow the rest to charge normally.

If you have "maintenance-free" flooded batteries, you may be able to pry off the cell covers to do this check.

With gell-cell or AGM batteries, you'll probably have to disconnect batteries one at a time to find the bad one. With the batteries disconnected, measure the voltage of each one to find the bad one - its voltage should be about 2 volts less than a good battery.

It is generally recommended that all batteries in a bank like yours should be the same vintage, and have the same usage history, so you may want to replace all the batteries at once, rather than one at a time as they fail.

You should be using "deep cycle" batteries (often called "marine" or "RV") for this application. Many boaters use 6 volt "golf cart" batteries connected as two 6 volt batteries in series, with several pair in parallel as needed for the desired capacity, for deep-cycle applications.


I will agree with Peter Bennett on the likelihood that one or more of your batteries may have gone bad. Being an electrician first and an engineer second, I would like to add a few things.

  1. It is a terrible idea to pry open a maintenance-free battery. Take it to a shop and get it tested, or buy (or build!) a load tester and learn how to use it. They're pretty cheap.

  2. The voltage on a bad battery - sometimes even a very bad battery - can still fall within ranges that look acceptable until you try to put a load on it. I've had this get me in trouble a few times, personally. See (1).

  3. You may want to check your MPPT charger to see if there is a ripple voltage. Its not likely, but you could be burning up batteries with a faint AC signal from the conversion. Just select 'ac volts' on a multimeter and put it across the terminals. Anything more than maybe half a volt needs to be investigated. Do this while the charger is running, with near full sun available - otherwise, you didn't really find out anything.

Other than that, I definitely second Peter's recommendation to keep all batteries in the bank at around the same age. Over time, the internal resistance will continually increase, leading to irregular charging and discharging cycles. Over years, you'll lose the capacity anyway, but having one completely die in the bank can cause reverse-charging, excessive gassing, and eventually, an explosion.

Also, you are definitely going to want to invest in a different pump. I don't know what the application is, but if it absolutely must have 13.5 volts, that means its marine/automotive, and built assuming you have an alternator running. There are other pumps that would handle this better. Furthermore, if its as small as I suspect it is, you may want to see if you can get an AC one and hook it up to that beefy-sounding inverter. You'll adding the overhead of the inverter to it, but AC machines are capable of being really quite efficient, so it might pay off.


A little late to the game here, and I believe that the above answers are the correct ones.

I would like to ask you to look at your panel "strength" and your battery capacity and how much energy your pump would use.

I don't know where in the world you are so how much sunlight you get or if your panels facing the "ideal" direction, but at 8 hours of FULL sunlight, no clouds, no lines loss etc, I.E under perfect conditions, it looks like it would take almost 2 days to fully charge your bank of batteries! That is without the pump running.

If you are not getting that much sunlight or there are a lot of clouds etc it could take a lot longer. And if you are using the batteries each day, then they may never get up to full charge.

Just a thought - unless my math is wrong!

Good luck


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