Solar charger / batteries question - Page 1
Forum section: electronics / beginners

author: jmh
author_rank: Contributor
timestamp: November 10, 2021, 08:51:55 am
content: Apologies for a noob-ish question that I should be able to answer myself.Scenario: one solar panel (not sure the wattage), two 12V sealed lead acid batteries wired in parallel. Occasional load of 250mA for a second twice a day (solenoid), a few times a year. (not my doing!). One battery measures ~12.1V and the other ~12.2V when disconnected. Everything works. V drops to 12V when the solenoid is activated and climbs back up.Apart from the issue of these batteries in parallel, my half-asleep reading this morning causes alarm bells as these batteries are being charged constantly from the solar panel with no control. They are a few years old to boot (no idea how many).So, first question: Am I being too paranoid here? It seems like a disaster in the making.We are suggesting replacing the solar panel (no mains power available) with a 'proper' one with a charger controller and new batteries.And, last question: presumably there are two batteries for resilience though I can't see how with them just in parallel. I would have though at least adding a diode per battery and connecting afterwards would be safer. But then charging both from a single solar panel would bypass the diodes. To me, a single battery is what they need, maybe with one disconnected to be swapped in in case the other fails - does that make more sense? (I reckon I can at least answer that question myself and this post is more a way of saying look what stupid did...! But I find I am questioning my reasoning because the setup has been that way forever...)

author: Zero999
author_rank: Super Contributor
timestamp: November 10, 2021, 10:29:36 am
content: If it's been working for years then what's the problem?Whether this is a good idea, or not, depends on the size of the batteries, the current and open circuit voltage of the solar panel.Does the solar panel have a built-in diode to prevent the batteries from discharging back into it, when it's dark?How about over-current protection? The batteries should have a fuse to protect against fire, in case of a short circuit.

author: fordem
author_rank: Regular Contributor
timestamp: November 10, 2021, 11:09:01 am
content: Let me start by pointing out that you haven't told us the capacity of the batteries, and that paralleling the batteries simply increases capacity, but does not provide redundancy or resilience. The voltages quoted suggest the batteries are, so to speak, undercharged, a 12V lead acid battery with a constant float charge would normally be over 13V and given a decent size battery, a 250ma discharge current for a couple of seconds would/should not cause a measurable drop in voltage.The lack of charge control, for me is a non-issue - I've seen commercial products (solar powered marine navigational aids) intended to operate unattended with the panel and the load connected directly to the battery, the charge control as such is managed by sizing the panel to suit the load - that said, I have also seen that very system overcharge the battery, and when I engaged the manufacturer, was told that it could not happen, it was self regulating - I provided the voltage measurements taken, and a couple of weeks later, the company vice president (responsible for engineering) called me, they had units installed world wide, and were experiencing the overcharge problem in only two countries, Guyana (where I was located) and the neighboring island of Trinidad.As I say it's for me, a non-issue, and since it's been working for years, I would say there's the proof.

author: jmh
author_rank: Contributor
timestamp: November 10, 2021, 11:20:43 am
content: Thanks both. I've not given much info because I simply do not have it. The batteries are probably (real finger in the air - I didn't note it down) 5Ah. I think the solar panel gives 18V off load.There are fuses in the circuit and I believe there is at least a diode in the solar panel feed.My only experience of similar batteries are in house alarms where they sit for years and then give nothing back when actually needed. But yes, this setup has worked for years and I am probably looking for problems that may never exist.So paralleling batteries (yes I realise this doubles the current) is not an issue - that was what initially concerned me.

author: Gyro
author_rank: Super Contributor
timestamp: November 10, 2021, 12:22:34 pm
content: A fully charged LA battery normally reads around 12.5V after being left open circuit for, say, half an hour. Your 12.1 - 12.2V indicates that they are not fully charged (maybe 50-70%). This implies that the solar panel isn't overcharging them (at least at the moment, I suppose it might be different in high summer).A load of 250mA for a couple of seconds a day requires virtually no remaining capacity to sustain. With the stated original capacity, they could be completely knackered and still continue to perform satisfactorily for years to come.The useful figure would be the battery voltage whilst still connected to the solar panel on a sunny day.

author: Peabody
author_rank: Super Contributor
timestamp: November 10, 2021, 07:09:09 pm
content: My understanding is that a lead/acid battery that is left partially charged all the time will sulfate up and die more quickly than one maintained at full charge. If true, that would explain why batteries don't last very long in Honda's Dual-mode charging system, which has been my experience.

author: Jwillis
author_rank: Super Contributor
timestamp: November 12, 2021, 05:43:00 pm
content: Any lead acid battery has a self discharge. This can be any where between 4% to as high as 10% per month depending on ambient temperature. The higher the ambient temperature the higher the discharge rate . So having a small constant charge on a battery that spends most of it's life "idle" is a good thing. In you case I would say that your charge controller is doing it's job . Even if your charge controller leaks a small current in an "idle" state then that's a still good thing . As long as your not getting an over charge condition . Battery sulfation is just a matter of fact for any lead acid battery and is accelerated in both under charge and over charge conditions . In your case I would say everything is working optimal and your batteries should have an exceptionally long life.