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theway · 10-11-2017, 15:34

A 100 watt solar panel 18.9v VMP is charging my 12v bank (2x6v AGM in series). The battery reads 13.8v or 14.7v or 14.2v based on the sun or charging stage. How do I calculate the amps going into the battery from it's charging voltage? (Without using a multimeter)

I've looked everywhere to find this, so I'm starting to think it's not possible or not as easy as it seems.

Thanks.

darylat8750 · 10-11-2017, 15:55

You haven't found it because you can't really can't do it. You need to invest in an amp meter.

**sailorchic34** · 10-11-2017, 15:56

Can't determine amps be generated or ah produced from voltage. Too many variables. You can figure 65% of rated amps as output in full sun (give or take 5%. So from 10 am to 3 pm you could get roughly 4 ah for each hour.

Why 65%. Well the rated output is done in a lab under perfect conditions. A solar panel aboard a boat will not be pointing at the correct angle. Plus clouds and shadows effect output. I've found that the 65% of rated amp capacity to be pretty close in the real world. OK on my boat anyway.

But if battery's have exited bulk charging then amp's passed through would depend on where the charge controller was during absorption phase. Then you're in PWM and ah output will drop as the battery's get close to full charge.

**newhaul** · 10-11-2017, 16:15

Quote:

Originally Posted by theway

A 100 watt solar panel 18.9v VMP is charging my 12v bank (2x6v AGM in series). The battery reads 13.8v or 14.7v or 14.2v based on the sun or charging stage. How do I calculate the amps going into the battery from it's charging voltage? (Without using a multimeter)

I've looked everywhere to find this, so I'm starting to think it's not possible or not as easy as it seems.

Thanks.

what charge controller are you using ? Most decent ones show the amps output from the panel.

theway · 10-11-2017, 16:19

Renogy 30amp. I think it's the model before they started the LCD screen.

But I'm actually curious to know if this is a calculation that can be done based on battery volts during charge.

**newhaul** · 10-11-2017, 16:33

Quote:

Originally Posted by theway

Renogy 30amp. I think it's the model before they started the LCD screen.

But I'm actually curious to know if this is a calculation that can be done based on battery volts during charge.

ok glad you are using a controller. Sorry you can't do it with math. Use the amp feature on the multimeter you are checking the battery voltage with. Btw the higher the battery voltage the lower amp. charge it will accept.

theway · 10-11-2017, 16:36

Awww darn. Ok, cool. Maybe I'll get the one with the screen

**newhaul** · 10-11-2017, 16:39

Quote:

Originally Posted by theway

Awww darn. Ok, cool. Maybe I'll get the one with the screen

here is the one I use on my boat will handle up to 400 watts.
https://www.windynation.com/Charge-C...628?p=YzE9MTc=
You can find it cheaper on eBay

SailsWithFists · 10-11-2017, 18:10

Your batteries determine the amperage flowing into them, and varies depending on the state of discharge.

NOTE: the following is highly simplified

A battery bank that is discharged below 70%~ will usually take the full amperage of the charging device. As the battery becomes closer to fully charged, it will reduce the amount of amps it is willing to accept.

If your battery bank is at 95% then the charging source regulator will still be allowing bulk phase voltage (14.6V usually), but the battery will only be accepting a small percentage of the charging source's amperage output. A battery bank that is 98%~ will generally be put into float phase (13.5V) and amperage will only trickle into the battery, no matter how much the source is capable of outputting.

So you can assume that if your battery bank is below 70%~ state of charge, then your solar panels will be delivering full amperage with direct sunlight perpendicular to the panels. Above 70%~ state of charge, then your solar panels will still be capable of delivering full amperage, but your battery will only accept a decreasing percentage as it reaches fully charged.

But as was said, if you want to measure it, you need an amp meter.

smac999 · 10-11-2017, 18:37

There is no way to calculate unless you know many watts you are producing.

If you are producing full watts from panel. Watts/ battery voltage will give you amps. But you will never get full watts.

So you measure voltage and current. And then you know how many watts you are getting from the panel

**newhaul** · 10-11-2017, 18:57

Quote:

Originally Posted by smac999

There is no way to calculate unless you know many watts you are producing.

If you are producing full watts from panel. Watts/ battery voltage will give you amps. But you will never get full watts.

So you measure voltage and current. And then you know how many watts you are getting from the panel

hence the amp meter to determine current (amps)

CSY Man · 10-11-2017, 19:07

Quote:

. Why 65%. Well the rated output is done in a lab under perfect conditions.

Try 110%...
Yeah, rarely seen, but a few times in the Exumas I have seen more than rated or advertised charging: 2 Siemens 75 amp panels, 4.4 ah each, total 8.8 max.
Midday in June, in the Exumas with clean panels and no air pollution I saw 10.5 ah on the Xantrax Battery Monitor. The peak only lasted a few minutes, but fascinating to watch, quiet cold beer and all.
(I also had a Blue Sky solar booster)