Solar Setup Advice Please?

Thread Starter


Joined Oct 7, 2016
I have a solar camping setup. And I have a supply of 75% ok 12v gel lead acid batteries the 21ah kind.

Needing as much power storage as I can get I thought I would be able to hook these 12v gell batteries up in parallel, obviously knowing in some form the bunch of batteries I have wired together will be pulled down by the battery with the lowest capacity.

Anyways - I hooked them up to my solar controller and they went flat - didnt charge at all. Controller output was from memory 14v. Its enough to charge the 12 100AH deep cycle lead acid I have. So first question is any ideas why?

Second question is I do need to build a form of BMS that can isolate these 12 batteries as if they were independant so one wont pull the charge out of the others. Is this a plausible project to take on?

Some ideas would be great.


Joined Sep 17, 2013
What does that mean?
I hooked them up to my solar controller and they went flat - didnt charge at all.
How many were connected in parallel?
Were you using them to drive a load at the same time as you were trying to charge them? If so, the load current exceeded the charge current. If not, one or more of the batteries is faulty and has a large leakage current exceeding the charge current.

Thread Starter


Joined Oct 7, 2016
In the old days, the battery specialist in the place I worked used to drain the cells down on a known resistance, time it, measure the current, and he would able to work out the battery capacity. These days, I have quite an expensive bit of equipment that tells me what the AH of a battery is within 60 seconds.

The 75% means these batteries are indicating they have between 16-24AH - new rated at 21AH. I dont think its fully accurate but they are 2 years old. So I reckon, sorry, long winded, 75% good.

I had 4 connected in parallel that time. No load except what is required to run the controller which is not much at all.

TBH I dont think they were charging at all. The solar controller does not put out a constant voltage, I believe it is more of a pulse voltage but I dont know. I do know, they went flat over the space of a couple weeks.

If I charge these batteries on my reasonably priced car battery charger, they accept the charge (it fails a few if they are not good) and I took a look at them today after 7 months sitting on the floor and they are still holding that charge. They just wont charge off my solar controller.

I am going to trouble shoot the whole lot more in the next few days. There will definately be some better than others, which is why I want to build a form of BMS so when they are all connected, that the crappy (lesser capacity) ones dont pull the rest down. I guess I may have to go arduino or some kind of monitoring programmable chip.



Joined Jan 29, 2010
hi b,
Try using a 12v vehicle lamp say 24W att or 36Watt as a load on your solar panel, dont have the batteries connected.
The lamp will load the panel, then check the solar panel voltage.
Lets know what you measure.

Thread Starter


Joined Oct 7, 2016
Thanks. I am pretty confident I will be able to work out the charging issue.

Its connecting 12-15 of these batteries is what my main priority is. Know what I mean?


Joined Sep 17, 2013
I do know, they went flat over the space of a couple weeks.
Well if the only load was the controller which is 'not much at all', that indicates one or more duff batteries.
Can you get the solar charger to charge just one of the batteries?


Joined Mar 10, 2019
It could be that 4 batteries in parallel have a much lower internal resistance than what the charge controller expects. That is, even one battery may be taking a lot of current, more than the charger can supply, and the charger shuts down. Even if all are about the same, asking a controller to charge 4 at the same time may exceed the controllers ratings or what it senses.
Try one battery at a time. Then pick what you think is a good battery (call it #1), and parallel another with it. See if it charges. Try the other 2 with this #1 battery. See if you can find which battery takes more of a charge when in parallel. If all batteries work ok in pairs, start with a third battery and see how the charger works. You may reach a point where you find a battery that is a bigger load than the others, or find a point where the charger/controller refuses to work..
Charging 4 batteries in parallel should be done so all batteries get about the same voltage/current. That means feeding the 4 around the middle of the battery bank, not at one end. That is, 2 batteries on one side of the feed point, 2 on the other side, all tied together with heavy wiring to reduce voltage drops.


Joined Jan 23, 2014
If you wanted to isolate the batteries, a rectifier in series with each battery would do the job. Turn the solar controller's voltage up by 0.7V or so to compensate. Or just let the batteries all sit unconnected for a while, measure their voltages periodically and see if one of them is dropping. Check them with a moderate load like a 27 watt incandescent car lamp. But first I'd want to hook up some instruments and monitor how much current the PV panel is putting out and how much is going to the batteries.

sparky 1

Joined Nov 3, 2018
A car battery can be difficult to pack down the road on foot.
The weight of 2 gel cells is light enough to pack. The use is simple at first and then they run down.
Now it's time to connect to the solar panel. Yes the solar controller is fine all is good.

A better system is to step up the voltage right after the solar panel to match the best practical needs
and at the same time the solar controller can safely switch in and out of charge mode.
At the battery the series connected battery voltage using step down is much more efficient.
A solar panel having 28V compensating step up improves the steady output in low solar condition.
The 24V controller further compensates and the step down to any voltage less than 24V has close to 98% efficiency.
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