Calculate Amps From Charging Voltage

[newhaul]

Here is the windy nation labeled model.
https://www.ebay.com/itm/P30L-LCD-30....c100033.m2042
Its $50USD but free shipping to the USA .
Worth a look.

[mitiempo]

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.

Theoretically the maximum output from a panel with a PWM controller is panel wattage divided by panel voltage - typically about 5.5 amps from a 100 watt panel.

With an MPPT controller divide panel wattage by charging voltage - usually about 14.4 volts.

The above assumes the battery will accept the current and panel perpendicular to the sun.

Best solution is a battery monitor. Not only will it show actual charging current but will give you pretty accurate battery SOC, which is more important. Especially with AGM batteries.

[john61ct]

A $20-40 meter will show volts and amps real-time.

At the higher end they do the watts calculation for you, and some even count accumulated AH over time.

Usually you just want to measure at the battery, how much is being accepted depends on SoC.

You can use that to verify your controller is holding Absorb voltage long enough before going to Float, by watching Amps decline, transition should be at 100% SoC, or endAmps = .5A per 100AH bank capacity (.005C) unless batt mfg gives a different spec.

Some people also use them to see panel output to the controller, which is of course a completely different set of numbers.

Using Anderson connectors, one meter can plug in to monitor multiple different loads and charge sources as needed.

A proper battery monitor to show SoC % accurately is much more complex and expensive.

Best is Merlin SmartGauge, easy to use and most accurate, but does not count AH.

For that I like Victron's 702-BMV, but lots more work is needed to get accurate SoC over time.

[SaltyMetals]

I use the Victron 702 monitor. Suits me fine. While it will give a lot of other info for charge/discharge amps/watts I usually leave the small screen showing %SOC. Boat has 3 x 50w solar plus windgen going through a hybrid MPPT controller and it's own monitor showing current/wattage contributions from solar & wind to 4 x 110Ah Victron AGM for services. The other battery circuit is 1 x 80 Ah Victron AGM for engine and 2 x 50 ah for bowthruster. Victron monitor seems to hv been a good choice.

[Seas the Day]

Originally Posted by theway

I've looked at a couple battery shops here in Bundaberg AU and the cheap 20a charge controllers with the screen are $120-$170!

Found a bunch 30A-40A on eBay quoted in AUD from $19- 85 and beyond including postage.

[newhaul]

Originally Posted by Seas the Day

Found a bunch 30A-40A on eBay quoted in AUD from $19- 85 and beyond including postage.

gotta be careful and check the specs to ensure it is programmable. Many in the cheap market are not and IMO are not worth the time to hook them up. Look for one that counts ah in and ah out on the load side.

[mitiempo]

Originally Posted by newhaul

gotta be careful and check the specs to ensure it is programmable. Many in the cheap market are not and IMO are not worth the time to hook them up. Look for one that counts ah in and ah out on the load side.

If you are referring to a solar controller I do not know of any that will give you output on the load side. It would be useless anyway as any loads do not go to the controller.

If you are referring to a battery monitor different story as they all do this, except for the SmartGauge.

[newhaul]

Originally Posted by mitiempo

If you are referring to a solar controller I do not know of any that will give you output on the load side. It would be useless anyway as any loads do not go to the controller.

If you are referring to a battery monitor different story as they all do this, except for the SmartGauge.

yes I am receding to the solar controller and most do count ah out if run through the load side of the controller. You have panels in, battery out, and load out connections . I have my refer running thru the controller right now to keep track of the ah it is using.

[hellosailor]

"How do I calculate the amps going into the battery from it's charging voltage?"
Ages ago the FCC used to have a famous question on one of their radio license exams about Ohm's law, and one of the answers involved "carrots".

Congratulations, you need carrots. [sic]

Actually, try:
https://smile.amazon.com/Keenstone-P...70_&dpSrc=srch

Inexpensive watt meters are commonly used in the R/C and electronics hobbies for this purpose. That one is a whole $15. They have varying capacities (total volts, amps, etc.) but almost all offer to track and record the number of amps and watts flowing from one side to the other. Install it in between the solar controller's output leads, and your battery, and you'll have a record of how much power has been put into the battery. The battery may lose 10% of that to charging inefficiencies, so it isn't a panacea, but it gives you a good start for minimal bucks.

[theway]

Then I guess the question now is what's better, a battery monitor or a better charge controller?

[john61ct]

If you *think* your SC is doing a proper job, you need at least a cheap V/A meter to confirm that is true.

If it is, then maybe you decide to splurge on an expensive proper SoC BM like SmartGauge to help you coddle your expensive bank.

But if you find your SC is a POS, spend your money first on getting a good one.

[newhaul]

A decent charge controller would be the starting place.