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Old 09-15-2014, 06:37 PM   #1
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More battery charge measurements information

I just returned from a 9 night trip, all boondocking, battery charge from 200 watts of solar, and towing charge. Two 6 volt golf cart batteries in series is my battery bank. That is about 1/3 more battery capacity than the original pair of group 24's Airstream supplied.

I have a Tri Metric monitoring system which tells me the % of charge via a semi sophisticated amp in vs. amp out measurement. I never dropped below 54% state of charge. This morning I was at 68% after a night of 35 F temps and a fair amount of furnace use.

I had about an hour and a half drive to home, and then the rig sat in semi sunshine for about 2 hours charging. When I put it in the carport, the battery charge indicated 80%. I plugged in the rig, and the Progressive Dynamics PD 4645 converter/charger took over.

What was interesting was that the charge when first plugged in was 11.5 amps. Since it is a 45 amp charger, I know that many would think it would charge at 45 amps. I have written about this in the past, that a high capacity charger does not charge at that high rate for very long as batteries simply will not accept the charge unless the voltage is very high, and then it damages the batteries.

So, at about 75 F, with an 80% charge on the batteries to begin with, the two golf cart type batteries would only accept an 11.5 amp charge from a 45 amp converter/charger.

This is also one reason that I don't feel that very large solar panel wattage sizes are necessary on RV's. Most RV battery banks simply won't accept high charge rates unless they are totally dead which should never be allowed. Now, if you are chronically parked in the shade, more panel capacity might help boost your solar output to what the batteries will take, but lots of panel capacity and limited battery capacity are mutually exclusive. Same goes for very high output converter/chargers. The batteries limit what they can take and a larger output one will not speed things up except on initial charge for a short period of time.
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Old 09-16-2014, 12:57 PM   #2
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Thanks for this. It is interesting and useful information. When I eventually get a solar system, I'm not sure whether I should start with 100w initially to keep cost down or go for the 200w which is my target system. I have two 12v batteries.
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Old 09-16-2014, 01:01 PM   #3
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I have 3 panels, 100,100,135 charging two Lifeline 6V batteries with 220 amp hours capacity via a SunRunner
Signature-25/6/Pro MPPT System from AM Solar.

I like the fact that if we're in partial shade there is still often charging by one or more panels seeing sun.
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Old 09-16-2014, 08:55 PM   #4
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Quote:
Originally Posted by cramar View Post
Thanks for this. It is interesting and useful information. When I eventually get a solar system, I'm not sure whether I should start with 100w initially to keep cost down or go for the 200w which is my target system. I have two 12v batteries.
I like the Grape solar panels you can get here in the states from Costco.com. They are 100 watts each and only cost $140 delivered to your door. However, I note that they come and go, right now the Costco.com listing does not have them, but I have seen that before and they returned shortly.
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Old 09-16-2014, 09:32 PM   #5
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How long had you owned your trailer before replacing the Interstate batteries? Also, did you need to replace them or was it just a matter of more capacity?
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Old 09-16-2014, 09:33 PM   #6
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Two months, more capacity.


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Old 09-16-2014, 10:57 PM   #7
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Quote:
Originally Posted by idroba View Post
I like the Grape solar panels you can get here in the states from Costco.com. They are 100 watts each and only cost $140 delivered to your door. However, I note that they come and go, right now the Costco.com listing does not have them, but I have seen that before and they returned shortly.
Grape is running out of in-stock panels, and their next shipment will most likely be subject to the new tariffs on Chinese solar components. We're holding our breath for now.

Also, part of the absorption charging phase is a reduction in charging amperage while holding the voltage steady. The amperage tapers as the battery fills. You will see this drop in charging amperage with any sophisticated automatic charger like the Blue Sky and Morningstar MPPT solar controllers and Magnum inverter/chargers.

And it looks like Progressive Dynamics too.
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