beginner mistake? charging small batteries in parallel

[joesuperstar]

Hey! I'm kind of experimenting but I don't want to cause a fire.

I'm messing around with some batteries. 💀⚡️I made a battery pack out of four alarm system batteries, each bat 7Ah sealed, in parallel at 12 V. My solar panels 100 W. I'm wondering if this is a bad idea. I don't know what could happen if I charge these batteries too fast.

What do you think? I'm clueless!
Any tips are very much appreciated!!!

[jony101]

as long as you have a solar charge controller you should be ok. The 100 watt panel at most will put out 5 amps at noon and should be floating by then. Your battery pack will be 12 volt 28 ah when you got them all connected together.
The recommended charge rate for a 7 amp battery is 1c (.7amp) but you can go as high as 3c (2.1amp) . The max for your battery pack at 3c is 2.1x4 = 8.4amps which is more than your panels power output.

You just want to watch the voltage and make sure it dont go above 14.2 volts during charging (your battery might have the max voltage on the top printed). If voltage is too high it will ruin the battery (it will cause the battery to gas and vent) , thats why you need a charge controller, it also senses when battery is full and switches from bulk charging to float charging . For a 100 watt panel all you need is a PWM controller.

Every charge controller I've had PWM or mppt has sensed the state of charge of my batteries and reduce amp output as appropriate.

[Sunking]

Not so much dangerous, more of a big No-No and a costly mistake. What is the battery Manufacture and part number? Being alarm batteries chances are high they are gel batteries which are the last thing you want to use.

If they are gel batteries even with 4 in parallel a 100 watt panel is going to dry the gel out very quickly from being charged way too fast. Once dried out they are toast. Basically they will not last and all that money you paid is gone.

[joesuperstar]

thanks

Thanks for the detailed message. You have no idea how helpful this is. I really appreciate it. Thank you!!

[joesuperstar]

Not so much dangerous, more of a big No-No and a costly mistake. What is the battery Manufacture and part number? Being alarm batteries chances are high they are gel batteries which are the last thing you want to use.

If they are gel batteries even with 4 in parallel a 100 watt panel is going to dry the gel out very quickly from being charged way too fast. Once dried out they are toast. Basically they will not last and all that money you paid is gone.

Good to know. Luckily, I have a surplus of these batteries. Plus, I'm usually running 50watt continuous, so, most of the time they're probably charging very slow.

[Sunking]

Good to know. Luckily, I have a surplus of these batteries. Plus, I'm usually running 50watt continuous, so, most of the time they're probably charging very slow.

That is not possible. 50 watts x 24 hours = 1200 watt hours. You have a 12 volt 28 AH battery system which means they only have 330 watt hours capacity and would be DEAD in 6 hours. Not to mention would take 400 to 800 watt solar panel.

At 12 volts running 50 watts continuous would require a 500 AH battery, or 71 of those 7 AH batteries.

[joesuperstar]

That is not possible. 50 watts x 24 hours = 1200 watt hours. You have a 12 volt 28 AH battery system which means they only have 330 watt hours capacity and would be DEAD in 6 hours. Not to mention would take 400 to 800 watt solar panel.

At 12 volts running 50 watts continuous would require a 500 AH battery, or 71 of those 7 AH batteries.

Sorry, I should have said I'm only running it in the day light. :/

[PNjunction]

Those alarm system batteries are typically agm, which can only handle 0.25 to 0.3C max. For a 7ah cell, that would be about 2.1a each, or for a 28ah battery, 8.4A. So you should be ok with the 100 watt panel, which typically is about 5a max under best conditions.

Yes, like some other references on the web, you CAN charge them with more current, but what they never reveal is that you drastically cut their cycle life. The chemical reaction is too fast, and they develop hot-spots within their structure. There is a reason that manufacturers limit agm's to 0.3C max.

The problem here is that you are playing with batteries that are of differing age and usage, so don't go nuts if they die early, especially if they have been sitting neglected for years in the office closet.

If you really wanted to get into it, charge each one individually and then put them in parallel for solar. To up your game with these small batteries, I'd recommend a Tecmate-Optimate charger since they test voltage retention after a full charge (amongst other tests). The Optimate 2, or 5 or 6 would be perfect. Plus, when you get tired of goofing on used paralleled alarm batteries, you can use these chargers on your vehicle. I do.