Charging while Draining a battery?

[BradKW]

Sometime ago I read something that seemed to indicate there might be an issue when using solar to charge the battery bank, while at the same time discharging it. 

It may be I misunderstood what was being said, but I've had this question in the back of my mind for awhile.

So are there any concerns?  ty

 

[rvpopeye]

That depends on the amount you are using.
The solar will put out what it can either way.
The amount it has left over if any to charge the battery is the concern.

 

[amwbox]

As long as you are pulling fewer amps from the battery than the charge controller is putting into them, they are still charging. If you pull more than the charge controller can supply (including efficiency loss from the charging itself), you are discharging them. Like when you charge a laptop while using it. It still charges, it just takes longer.

Only issues I can see are with batteries which need a higher amperage to properly charge than the net amperage left after whatever you are drawing. And if you are drawing enough to prevent them from reaching a full charge, obviously that has negative consequences over time.

 

[Optimistic Paranoid]

Don't really understand the question.

If the sun is shining on the solar panels, and the panels are producing a higher voltage than the battery voltage, then they will be charging the battery.

If you are also trying to run something electrical at the same time, then it will all depend on the amps involved.

If the panels are producing X amps, and the load is trying to draw Y amps, then:

if X is greater than Y, the battery will be charged by the difference between X and Y.

Example:  If X is 12 amps and Y is 10 amps, the solar panels will power the load and the battery will get 2 amps of charging current.

If Y is greater than X, the panels will be unable to power the load on their own, and the battery will make up the difference.  The battery will be discharged by Y minus X.

Example:  If X is 12 amps and Y is 14 amps, The solar panels will send ALL of their power to the load, and the battery will make up the 2 amp difference.

 

[BradKW]

Only issues I can see are with batteries which need a higher amperage to properly charge than the net amperage left after whatever you are drawing. And if you are drawing enough to prevent them from reaching a full charge, obviously that has negative consequences over time.

Thanks, that rings a bell and I believe that was it. I saw it back before I understood what AGMs actually were and that they want the high amp kick, and that running an AC prevents that even if you're still charging batteries.

 

[rvpopeye]

975 watts @24 volts should be a very decent amp output into that 12volt battery from a mppt controller , They'll usually be getting a nice charge rate !

 

[WanderLoveJosh]

i'm experiencing this right now.. just hooked up 2 6volt batteries to my 300w panels... and granted its late in the day, but at one point it said i was taking in 2.7 amps.. and went up to 53% SOC, then as the clouds came, it went back down to about 40% which is where it is sitting. My inverter isn't on, but is connected to the battery. I'm worried about waking up with a completely wiped out bank, especially since I just got these and haven't gotten a chance to charge them fully.. wonder if I should disconnect them and wait for the sun tomorrow?

 

[highdesertranger]

I would unplug them just to be on the safe side. but the bigger question is how did the battery state of charge get so low if you just hooked them up? another point, what are you using to determine your state of charge(SOC)? highdesertranger

 

[WanderLoveJosh]

When I plugged them in it first said 50% from the renogy Adventurer charge controller, then it went down as a big cloud came, once I could see it going away, i moved my van to a spot where there were no trees, and it went up too 53% then back down. I'm sure had I been able to do this at like noon, it would have fully charged super quick. I'm just wondering if since I didn't have the batteries plugged in where they could fully charge, if I should go ahead and unplug them then wait till the sun comes out tomorrow?

 

[highdesertranger]

did you program the charge controller to the ACTUAL SOC or just plug it in? you need your batteries at 100% SOC and then program the controller to 100%. with flooded batteries you need a hydrometer and a voltmeter, to get a true reading. highdesertranger

 

[BradKW]

Sounds like something is wonky with your charge controller, or just that it's not set up correctly yet, because without any load there's just no reason why the SoC should fluctuate like that simply by moving into shade. As long as nothing is draining the battery, it'll be fine until tomorrow..

 

[SternWake]

The SOC display on these controllers is not to be trusted.  It is entirely voltage based and voltage alone can't EVER represent state of charge on a battery that is in use, either being charged or discharged.

The state of charge display provided on these controllers is practically criminal for the confusion it will cause, as it is doing in this scenario.

There are 3 ways to know when a battery is indeed fully charged.

A hydrometer, an Ammeter and a voltmeter, and an amp hour counting battery monitor.  When amps required to hold absorption voltage taper to a certain level then it can be assumed 100% has been reached.

There is no way a voltage based state of charge indicator can be anywhere near accurate on a battery that is still in use, being charged or discharged.

Ignore it.  it is a useless bell and whistle.  Get your battery upto 14.x volts for 4 hours, then test specific gravity on wet cells or measure amperage required to maintain absorption voltage if an AGM battery.

Or get a real battery monitor with Shunt wired correctly and programmed correctly, like a trimetric or a victron..

 

[WanderLoveJosh]

Yeah now I'm def starting to think its unreliable because today it was at 100% SOC.. Get back to where we park, where its a little bit shady and it drops down to 40%.. Now it's slowly working up since there is some sun hitting it. Ughhh such a pain in the assss. I don't get paid till next week, so I guess just stay parked and plug into shore power which we usually do, and let it do its thing.. If I'm not running off the batteries then they should be fine right?