Charge batteries from charger and solar at once

rgk1 · May 2011

Can you have batteries being charged from a battery charger (Iota) and solar charge controller (Morningstar SunSaver) output at the same time? My battery cable +, charge controller batter +, and Iota charger + are all tied together on the line side of the main DC breaker. The - of each are also all tied together at the shunt. May be a silly question, but can you mix the different output voltages together?

dwh · May 2011

Re: Charge batteries from charger and solar at once

You can think of it like an air compressor. The battery is the tank and you have two different compressors connected to it. One puts out 13.6 psi and the other puts out 14.4 psi.

As long as the tank pressure is below 13.6 psi, then air will flow from both compressors into the tank, but once the pressure in the tank gets to 13.6 psi, then air is only flowing from the 14.4 psi compressor.

The problem to watch out for, is not the voltage (psi) it's the current (cfm). If the battery is below 13.6v, then you could have full output current (bulk stage) from both, which might be more than is good for your battery.

For example, if your battery recommends 30a max charge current and both of your chargers are rated at 15a, then no problem - but f your chargers are both 30a units, then you could hit your battery with 60a which would be double what is healthy for the battery.

SolarT · May 2011

Re: Charge batteries from charger and solar at once

An option, put the Iota on a timer that turns on at night only. Solar charge during the day.

rgk1 · May 2011

Re: Charge batteries from charger and solar at once

The charger is a 45 amp and the solar is 15 amp max. I have 220 AH of AGM's. I will have to see what amperage the agm's can take or do as SolarT says and use the charger at night if needed and solar in the day.

mike95490 · May 2011

Re: Charge batteries from charger and solar at once

If you have a place to plug the charger in, why are you using batteries.
Generally, your best bet is to use the charger/generator in the AM, and bulk the batteries early, and let the sun do the slower part of the charging. saves fuel, unless you need to run the genset at night to cook with the microwave.

rgk1 · May 2011

Re: Charge batteries from charger and solar at once

My setup runs lights and small items in the shed and a dusk to dawn light at night outside. Solar keeps up most days. I use the charger to keep it up on heavy use days. I will also use it on the home to power lights, clock, satellite and tv during an outage. The main senario for the question was: I run the batteries down during the overnight during and outage and either the power is back on by morning or hook up the generator to charge the next morning. If the sun is shining next day and the solar kicks in, what happens with both of them charging together. I just didn't want to have to "babysit" it if both charging at the same time wasn't ok.

2manytoyz · May 2011

Re: Charge batteries from charger and solar at once

I have a 75A Iota charger connected to my battery bank, along with 630W of solar panels (900 by the weekend).

There's no problem having multiple chargers (and types) connected to the battery bank. They each are self regulating, and will taper off as the battery voltage increases. One charger will also not back flow into the other.

But leaving the charger connected and powered up means any battery energy used, will be replaced by grid power. Especially when using battery power at night. By the time morning rolls around, your battery bank will already be at 100%, and the solar panels will have almost nothing to do.

My solar array is large enough to keep up with my loads most of the time. If we have a major outage, and I end up running both fridges on battery power overnight, or have bad weather for a few days, I'll need to plug in the Iota charger to bring the bank back up to 100%. FWIW, the 75A Iota charger works great with my Yamaha EF2400iS generator.

Details about my setup here: http://2manytoyz.com/iotacharger.html

rgk1 · May 2011

Re: Charge batteries from charger and solar at once

Thanks for the info. I was afraid if one with higher voltage or I guess amperage, may try to backflow to the wrong location.. not the batteries. My solar almost always keeps up with my small needs. Its the occasional outage that will drain the batteries. I have a secondary motive for having the charger that I have not yet asked here in the forum and have been researching instead. But since I am on the subject, can someone tell me the relationship between amps and voltage into the battery bank to fully charge and possibly equalize on the agm's. I would love to have twice the battery bank for an outage. I currently have 220 ah. I can justify the cost of the batteries, but not the thousand or so for more solar and larger charge controller that would be capable of that size battery bank. I was hoping to use the Iota charger to bring the larger bank back up to full charge and then let the Morningstar controller once again keep up with daily use. Is that possible, or would I need twice the amp input capacity?

Cariboocoot · May 2011

Re: Charge batteries from charger and solar at once

I'd advise against expanding your battery bank without some significant increase in charge capacity. The reason being that at 15 Amps max you're at about 7% charge rate of the existing bank. Unless your usage is low and you spend a lot of time in 'float' daily, you really don't have much capacity for charging and handling loads as is.

You could use the Iota for bulk charging and then 'finish off' with the solar, but it would be a real pain in the anatomy to do so every day.

BTW, you don't equalize AGM's unless the manufacturer specifically says so. Just leave them be and use a battery monitor if you have one.

Cariboocoot · May 2011

Re: Charge batteries from charger and solar at once

Post Scriptum;

You could separate the two banks with a battery switch, and leave the "emergency" bank alone - giving it a boost charge now and then with the Iota. AGM's have a fairly low self-discharge rate and can be left to their own much longer than an FLA.

Still a nuisance, though, as you would have to look at the Voltage level of the "emergency" bank periodically or else set it up so that whenever you run the generator it automatically gets recharged.

BB. · May 2011

Re: Charge batteries from charger and solar at once

The practical rate of charge for a lead acid battery bank works out to around 5-13% times the 20 Hour rate.

Below 5%, for flooded cell you have self discharge and electrolyte mixing issues.
For all types of lead acid batteries, you want to recharge them quickly above ~75% state of charge. If you have a low charge rate, it could spend days below 75% state of charge and tend to sulfate the batteries (for an early death).

For flooded cell, standard storage batteries charged >~13% rate of charge, can over heat (with quick charge industrial chargers). For solar panels, they are expensive and >13% rate of charge--they are not going to charge at that high of rate very long before the battery voltage rises and charge rate tapers off.

For Concorde AGM, they can charge at less than 5% rate of charge (no electrolyte to mix, lower self discharge). And they can charge over C*4 rate of charge (check vendor manuals).

For standard utility powered charger (and gensets to a degree), you can have a charger rated upwards of ~25% rate of charge without concern.

Basically, the way many chargers work:

Bulk (less than 80% state of charge): Charger outputs rated current into battery bank
Absorb (~80-90% to 100%): Battery reaches charger set point (such as 14.5 volts) and charger will taper off on current to keep 14.5 volts
Float (100% state of charge): When charging current falls to a few percent of battery AH capacity and/or a timer (~2-6 hours) is reached in absorb mode--The charge controller will cut back to ~13.2-13.6 volts to keep the battery charged (self discharge--also differing forms of corrosion occur in a battery depending on voltage of battery--13.x volts is a compromise for long life between the various failure mechanisms--and to reduce water consumption).
Equalize: Done on 100% charged battery. Increase voltage to ~10-16 volts and 5% rate of charge. Done for flooded cell batteries roughly when needed / once every couple months. Hold ~5% rate of charge and monitor specific gravity of each cell until SG no longer rises in any of the cells (check every 1/2 hour).
Equalize AGM: In general, many vendors do not recommend equalization--other recommend a "light" equalization (14.2 volts or so held for a few hours?) to equalize the level of charge among all cells perhaps 1-2x per year. AGM's, if over charged can overheat and release pressure (gas+electrolyte)--which will shorten their life.

I think I got the above correct... As always, read the manuals for your battery brand/type for their recommendation.

-Bill

dwh · May 2011

Re: Charge batteries from charger and solar at once

And FYI:

The Iota IQ/4 module is okay for AGMs because it doesn't actually equalize. What the IQ/4 does, is after 7 days as float, it triggers bulk stage and does a quick run-through of the bulk and absorb stages and then it's back to float.

They call it equalize, but it really isn't.

demobanjo · June 2016

Thanks! I found this useful. I had a 40W solar panel for charging my batteries with a 7A solar charge controller. I just upgraded to a 120Watt panel and 30A solar charge controller. I was wondering if I could leave the 40 Watt Panel connected to the battery (200AH, 12V).

From the discussions above, It does not hurt the batteries because each panel only have a max of 6A for the 120W and 2 A for the 40 W panel. 8A is still safe for my battery.

BB. · June 2016

Yes, wiring each controller+array (properly) back to the battery bank is fine (each controller should also be properly configured for the correct voltage set points, etc.).

It is possible to put "too much" charging current into a battery bank... For flooded cell batteries, around 13% of (20 hour) Amp*Hour rated capacity is good for solar... Not much more than 20% rate of charge for a generator powered AC charger.

Either way, when you get much above 13% (or are equalizing your battery bank), it is possible to overheat your battery bank (hot batteries are never a good thing--Ages the battery bank faster, and high charging current/temperatures can result in thermal run-away for the batteries--Hot batteries, charging voltage drops, charge controller pumps in more current, temperature rises more, voltage drops more, etc.).

-Bill

Charge batteries from charger and solar at once

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