Battery voltage discrepancy

Eric L

Hi Folks,

I'm a newbie to solar with a small system that powers a small 28w DC pump. When I measure my battery voltage with a multi-meter, I always get a lower value than I do from my charge controller. The controller is a Morningstar Sunsaver MPPT 15, and using MS View it always shows the battery voltage to be about .20 v higher than the multi-meter. This is the case even at night, with the PV panels off and no load on the batteries. So if the multi-meter shows 12.53 volts, the charge controller says 12.73.

Hydrometer readings are closer to the Sunsaver reading, although I should add that my hydrometer is one of those cheap floating plastic disc types.

I suppose the multi-meter might simply be poorly calibrated, but is there any other explanation for this discrepancy? I'm taking the readings with everything still connected to the batteries, in case that makes a difference.

The rest of the system is 2 Kyocera KD135 panels in series and two Trojan T-105 batteries in series.

Thanks for any suggestions, and I've learned a lot from this forum.

icarus

Re: Battery voltage discrepancy

Put new batteries in a DVM, and try it again. Then borrow another DVM and compare the two. I would suspect the calibration of the DVM, and possibly the MS controller. It is also quite possible that you have a voltage drop between the controller and the battery. Look carefully at the cables and the connections. Can you test the output of the controller at the controller as well as at the battery?

Good luck and welcome to the forum.

NEOH

Re: Battery voltage discrepancy

Discrepancy? Maybe, maybe not ...

Let's say the battery voltage is actually 12.63 Volts. Then the Charge Contoller is within +1% and the DVM is within -1%. The 12.53v DVM and 12.73v CC readings may be totally acceptable given your equipment.

What is the stated accuracy of your Charge Controller and DVM ?

waynefromnscanada

Re: Battery voltage discrepancy

NEOH wrote: »

Discrepancy? Maybe, maybe not ...

Let's say the battery voltage is actually 12.63 Volts. Then the Charge Contoller is within +1% and the DVM is within -1%. The 12.53v DVM and 12.73v CC readings may be totally acceptable given your equipment.
What is the stated accuracy of your Charge Controller and DVM ?

Agree. I have the same controller on my mini-micro hydro and find the same thing. It's withing reason, in the ball park. Just the length of wire between the controller and the battery can do that as well. Not something to worry about at this point.

Eric L

Re: Battery voltage discrepancy

Thanks for the replies.

Icarus, I did measure controller output at the controller with the DMM and it was identical to the DMM's battery measurement. The battery cables are short (about 16"), and #4 AWG, so hopefully no noticeable losses there.


NEOH, the stated accuracy is 1% for voltage on the controller, so what you and wayne say sounds plausible. So yeah, the discrepancy may be a nothingburger, as you suggest.

I'll infer from the replies that there is nothing wrong with measuring with the DMM with everything hooked up to the batteries, which was one of my doubts. Still, I'm seeing the advantage of a nice quality hydrometer. Right now I feel like the guy who has two watches and so is never sure what time it is.

hayray

Re: Battery voltage discrepancy

I have a cheap DVM that I bought at Walmart for $10 and its reading is identical to my charge controller read out all the time. Good chance there is a voltage drop or controller is off. I think ,2 volt is a big difference at least for me. I am running 2, 3 amp DC pumps and I am always checking to make sure I have enough battery life. If I go from 12.4 down to 12.2 then I am getting down to almost 50% charge and that would be a big deal.

icarus

Re: Battery voltage discrepancy

Hayray,

You do realize that voltage testing under load is not a good way of judging battery SoC?

An accurate battery voltage can only be obtained when the battery is neither being loaded nor charged, and then is allowed to sit idle for several hours. Only then will voltage be a good indicator of SoC.

http://www.batteryfaq.org/

Tony

hayray

Re: Battery voltage discrepancy

I am not referring to SOC, that is another reading on my controller also. I also check the voltage right in the morning before any sun. But he should still not have that difference in voltage between measuring devices. Yes, I do realize that voltage, especially under load or under charge is pretty meaningless for some part. As soon as I start getting a good charge the voltage jumps right up eventhough the battery has not had any major charge time. Thanks for the info.

NEOH

Re: Battery voltage discrepancy

hayray wrote: »

But he should still not have that difference in voltage between measuring devices.

Given that the DMM and Charge Controller each have a +/-1% accuracy then do the math and tell us what the allowed range of valid readings are allowed to be ...

phred01

Re: Battery voltage discrepancy

Eric L wrote: »

Hi Folks,

I'm a newbie to solar with a small system that powers a small 28w DC pump. When I measure my battery voltage with a multi-meter, I always get a lower value than I do from my charge controller. The controller is a Morningstar Sunsaver MPPT 15, and using MS View it always shows the battery voltage to be about .20 v higher than the multi-meter. This is the case even at night, with the PV panels off and no load on the batteries. So if the multi-meter shows 12.53 volts, the charge controller says 12.73.

Hydrometer readings are closer to the Sunsaver reading, although I should add that my hydrometer is one of those cheap floating plastic disc types.

I suppose the multi-meter might simply be poorly calibrated, but is there any other explanation for this discrepancy? I'm taking the readings with everything still connected to the batteries, in case that makes a difference.

The rest of the system is 2 Kyocera KD135 panels in series and two Trojan T-105 batteries in series.

Thanks for any suggestions, and I've learned a lot from this forum.

Both multimeter and the charge controller can be out. Generally the accuracy is quoted say =/- 5% so one can be +5 and the other -5 thye diffrence in the extreme can be 10%