Solar charging - question (2 batteries, 1 controller)

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Solar charging - question (2 batteries, 1 controller)

Postby Doctor Pancakes » Wed Aug 25, 2010 11:57 am

Can I safely charge 2 differing batteries simultaneously on the same solar charger? Both batteries are 12v but ...

One battery is an 80Ah deep-cycle battery that is permanently connected to the system & serves our camp needs; the other is a 20Ah sealed lead-acid battery I remove at night to illuminate my bike, then want to recharge by day.

Question: Any safety (or other) concerns with just hooking the SLA battery in parallel with the deep-cycle one by day to charge it, given their different chemistries and charge levels?
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Re: Solar charging - question (2 batteries, 1 controller)

Postby uski » Thu Aug 26, 2010 9:27 am

Doctor Pancakes wrote:Question: Any safety (or other) concerns with just hooking the SLA battery in parallel with the deep-cycle one by day to charge it, given their different chemistries and charge levels?


DO NOT EVER do this UNDER ANY CIRCUMSTANCE.

If you connect the two batteries in parallel, the one which is the most charged will discharge to the other one without any current limit. If they are both charged almost equally it might be OK, but otherwise this is a fire hazard (really) and it will definitely shorten the lifespan of your batteries if they don't just burn instantly.

There is no easy way to charge two batteries from one solar panel with the cheap controllers.

Most of the time they just connect the solar panel to the battery with a over-charge cutoff, the current being limited by the solar panel, so you can't even add another controller in parallel with the solar panel (it will charge the battery which is the most discharged first, and will then charge both, at the rate of the battery which is the slowest to charge).

So adding another controller might work if they don't try to be too clever but I recommend charging only one battery at a time.

What is the controller you are using and your solar panel power BTW?
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Postby FIGJAM » Thu Aug 26, 2010 10:31 am

Even with a regular battery charger Ive been told not to charge batteries that arnt matched.

Hooking up batts together that are not the same amp hour and type will screw up your batteries.
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Postby mthyer » Thu Aug 26, 2010 11:37 am

Usually you need an isolator or some other device to seperate the batteries and prevent the fire hazard. Some charge controllers can act as an isolator, but they'd likely have two charging pairs and instructions for setting up each battery in an isolated circuit.
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Postby Token » Thu Aug 26, 2010 12:26 pm

I'll disagree with all the doom and gloom crowd here.

I've connected a 400Ahr battery to my car battery which was drained at the time, with jumper cables and nothing caught on fire. I was able to start my car and recharge the battery via the alternator.

All lead acid batteries have an internal resistance. When you connect two in parallel there is some seepage before the voltage is equalized UNLESS there is a 13.8V DC source dumping current into the circuit. Then its just a parallel resistor network.

Note that a fully discharged lead acid battery still has over 10V on it.

You can connect both batteries in parallel but make sure you use a thick cable.

The solar will charge both at the same time. If one is smaller, it will just present angreater resistance and get less current.

The system will balance itself.
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Postby gyre » Thu Aug 26, 2010 1:17 pm

Internal resistance is the bigger issue and can vary wildly.

I'd charge them separately.
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Postby uski » Thu Aug 26, 2010 3:59 pm

Token wrote:All lead acid batteries have an internal resistance. When you connect two in parallel there is some seepage before the voltage is equalized UNLESS there is a 13.8V DC source dumping current into the circuit. Then its just a parallel resistor network.


Lead-acid batteries have a very low internal resistance. This is what makes them able to supply very large currents.

I maintain there is a fire hazard (or, at least, a "battery damage" hazard) If you connect a discharged battery to a charged battery directly, because the discharged battery will have a relatively high internal resistance and you'll be essentially dumping power into it, transforming the battery you want to charge into an electric heater. And batteries don't like heat very much, nor they like to be charged too fast.

Token wrote:You can connect both batteries in parallel but make sure you use a thick cable.


This recommendation shows that you know that there is a high current moving between the two batteries, and that cannot be a good thing. Batteries are not supposed to be charged at high currents, especially when they are deeply discharged, for the reason I explained above.

Token wrote:The solar will charge both at the same time. If one is smaller, it will just present angreater resistance and get less current.


Yes, this will work once the batteries are balanced, but doing the balancing just by connecting the two batteries together really seems like a bad idea to me.

But maybe it's just me...
...gotta finish packing! :)
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Postby Token » Thu Aug 26, 2010 5:14 pm

What better place to explode an acid battery than an alkali lake bed!

:shock:
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Postby Doctor Pancakes » Fri Aug 27, 2010 9:34 am

Thanks for the tips guys. Some battery gurus on another forum suggested a large (12v, 50w) light bulb in series (between the positive terminals) to moderate the current. And the consensus there was: This wouldn't be a great idea with a larger system but probably wouldn't be a problem for one my size.

I don't expect my power needs to be great, anyway, so I'll charge the batteries separately when possible.

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Postby FIGJAM » Sat Aug 28, 2010 9:04 am

If I hook 2 battery chargers to one 12 volt battery, does it just double the amps or does it create a 24 volt charger?
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Postby gyre » Sat Aug 28, 2010 9:33 am

Depends.
Unwise.
Won't create 24 volts.
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Postby FIGJAM » Sat Aug 28, 2010 10:07 am

102ah agm. The combined chargers will provide 12amps so that I wont have to run the genny as long.

Thanks gyre: Im so busy packing I just cant think in tech right now. :roll:
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Postby Just_Joe » Sat Aug 28, 2010 10:15 am

My solar charge controller manual tells me that it's OK for it to work "together" with the charge voltage from my RV inverter.
I've doubled up a normal 110V 3 amp trickle charger with my RV inverter/charger and haven't blow up yet.

Fig- as long as you connect all of the positives together and all of the negatives together, you're should remain in the 12-14 volt realm.
If you hook positive from one charger to negative of the other charger, the remaining two leads will equal 24 volts. THAT'S something I'd be 'scaird of.

*caveat*- I don't think I've ever had a deep cycle battery that lasted more than 2 years and have had more than my fair share of acid burns ...
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Postby FIGJAM » Sat Aug 28, 2010 10:45 am

Thanks weather man: Yea I knew if they were hooked in series it would create 24volt. I just wanted to double my charge rate. Looks like it will work alright.
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Postby Foxfur » Wed Jun 15, 2011 7:35 am

I've worked with industrial UPS's and gel cells / AGM's extensively. The biggest had 100 200Ah AGM's divided into 10 paralelled packs of 10 batteries in series. These 120VDC parallel packs all charged simultaneously. Granted, the packs are typically discharged similarly but there can be variances of up to 20% and the manufacturer's info says they're fine with it.
I have two 25Ah and 6 11Ah gel cells that I keep on a single 2amp charger for maintaining charge. They've been there for 6 years. When I use one to run lighting and radios, I put it back in the bank to charge with zero issues. No fire, no smoke. They're in the living room corner. Courting disaster? Perhaps. Experience dictates otherwise.
YMMV.

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Postby Trishntek » Wed Jun 15, 2011 8:33 am

One thing about batteries and electricity is you usually don't have to wait very long to find out whether you did it wrong!
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Postby capjbadger » Wed Jun 15, 2011 12:52 pm

"2 batteries, 1 controller"... lmao

Really? No one? :shock:

The easy route here is get another charge controller. You only need a cheepie and they can be found on ebay all the time for around 7 bucks or so.

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Postby gyre » Wed Jun 15, 2011 1:02 pm

Trishntek wrote:One thing about batteries and electricity is you usually don't have to wait very long to find out whether you did it wrong!

I have a new and healthy respect for batteries after seeing the photos of the destroyed shop from when the scooter kit I have's battery pack exploded during the charging cycle.

It was a gorgeous plexiglass and round battery setup with ample venting and cooling, in nicad or nimh.
Glad I wasn't sitting on it then.
Would have loved that pack.

Lest anyone get cocky, the designer is a career research scientist.

They were doing interviews with a chinese researcher in electric vehicles.
Turns out he has rebuilt his shop several times.
And he uses lead batteries, much more tolerant, I think.


Boom!
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Postby BBadger » Wed Jun 15, 2011 3:56 pm

You should connect a large rectifier diode in series with each battery to ensure that charge can only flow into the battery, and not out of it. The solar panel charging circuit itself has one to prevent battery from discharging into the solar panels the the solar panels, which could heat them up and fry them.
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