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Old 15-02-2016, 15:40   #1
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Battery Charging Questions


We recently purchased our boat that had well used AGM 8 D batteries...or they were not properly cared for.

We have converted to flooded lead acid golf cart 6 v batteries with a total ah capacity of 940 ah. I spoke with the battery company and got the bulk and absorb charge levels at 14.5 volts, and the float charge at 13.5 volts. I reprogrammed the prosine 2.0 inverter charger to match those levels. We also added 2 independent solar panels with each a separate mppt controller that are also programmed to the battery specifics.

When I charge my batteries from either the prosine charger or each charge controller, the charge levels are not locked in at 14.5 or 13.5 v. I see anywhere from 14.4-14.9 v for a bulk or absorb charge and 13.3-13.7 v for the float charge. I have battery sense lines connected directly to the batteries and the sense lines agree with the charge levels within about 50 mV.

My assumption was that the charging systems (either charger or mppt controller) would lock in at the specific programmed voltage and deliver the current as needed at those voltage levels. Why doesn't that happen?

Thanks for your comments.

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Old 15-02-2016, 16:38   #2
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Re: Battery Charging Questions

Both my outback solar controller and my magnum charger inverter charge at a steady set voltage.

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Old 15-02-2016, 17:25   #3
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Re: Battery Charging Questions

For LA batteries bulk charging is typically a constant current mode and the voltage will rise during the charge cycle to keep the constant current flowing. Acceptance and float are constant voltage and typically temperature dependent. Off the top of my head the proper float voltage can vary by over a volt depending on the battery/ ambient temperature.
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Old 15-02-2016, 17:52   #4
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Battery Charging Questions


Do you have a temperature sensor on the battery bank connecting back to your main charger, or did you work out temperature compensation when you established your bulk and float set points?

That could be one factor contributing to the variations you are reading- depending upon the ambient temp where the batteries are mounted, and their temp after some time under bulk charge.

Out of curiosity, which battery brand and model are you using? e.g., we have Trojan T105 six volt batteries and at 77 degrees F the manufacturer set point recommendations are: 14.8 bulk; 14.2 acceptance, and 13.2 float (from memory- I'm not on the boat at present and would need to confirm the acceptance set point...)

It will be interesting to learn more.



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Old 15-02-2016, 18:11   #5
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Re: Battery Charging Questions

I think your statement swaps the concept of a steady state voltage (or end result)
for what are your observed initial/transitional states.

In other words, there is a process during the charging and you start with low batteries. It takes a while for the batteries to charge up, which is at some point limited by the algorythm of your smart controller. The voltages you talk about are merely the end result which indicate when the charger should reduce the amount it delivers, to avoid over charging.

Here is a pretty good post:

I think your observations are what you would expect to see, that you are not seeing a problem. You're at the point where the explanation gets tough for sales teams, and support engineers as well, perhaps for different reasons ;-)

Hope that helps.
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Old 15-02-2016, 18:37   #6
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Re: Battery Charging Questions

For the OP....

Most likely the small variations in float voltage you are seeing are the result of intermittent loads (frig, other onboard loads) AND intermittent charging (solar, alternator, etc.).

On my boat with six golf-cart batteries for the house bank and a high-end smart charger (Victron MultiPlus) programmed to my liking, I see the same sorts of small variations. Even changes in the AC loads can cause some chargers to "begin all over" or to "reassess" where they are in the charging cycle. For example, when I've been on dockside power for some time with the batteries fully charged and on float, if I switch on a significant AC load (like an air conditioner or heater), the Victron will revert temporarily to absorption voltage.

I'm pretty sure this is what's happening to you, and have the "proof" here in my basement. I brought the six golf-cart batteries home for the winter and they're on 24/7 charge and float at 13.8VDC. There is no load on these batteries, and I've never seen a variation from the 13.8VDC float level I chose. By the way, these six Crown CR-235s are 235AH each and are now 2.5 years old. On this constant float, they absorb a grand total of 4 watts or 0.29A.

Note that manufacturer's charging recommendations vary considerably, even for the same battery from the same manufacturer. I think some are written by sales people, or lawyers, or technical writers or, in rare instances, by engineers. Trojan, for example, gives several prescriptions for charging and floating their flooded batteries. There is no explanation for the variations.

Over many years of use of T-105s, testing of AGMs and gels, and working on client's electrical systems I've come to the belief that for good quality flooded batteries like Trojans and Crowns a good regime is:

Bulk -- 14.8 VDC
Absorption -- 14.8-15 VDC
Float -- 13.8 VDC
Storage -- 13.2 VDC

Periodic repeat absorption -- 14.8 VDC for 30-60 minutes every 2nd day

Equalization - 15.5-16.0VDC (rarely, maybe once or twice a year)

Note that if you provide these voltages the batteries themselves will decide how much amperage they will accept.

Note also that this somewhat aggressive charging regime is in part intended to keep sulfation and stratification at bay, and has been shown in my research to prolong the life of the batteries.

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Old 15-02-2016, 18:38   #7
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Re: Battery Charging Questions

Thanks for the replies. I do have temp sensors attached for both the prosine and the mppt controllers (Morningstar).

I thought I understood what was supposed to occur, but maybe I need to study the topic more.

I'm not sure it's well know by most. I asked a couple vendors at the boat show in Miami and they couldn't explain it.

Battery science ...tricky stuff. Thanks for any insight.

Btw...the batteries are Crown 235ah
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Old 15-02-2016, 20:05   #8
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Re: Battery Charging Questions

Have a look at the controller manual, some controllers have an "equalization" setting which is set higher than the bulk or float settings. The equalization charge is supposed to stir up the electrolyte occasionally to get rid of stratification. It's only supposed to kick in every couple of weeks or so.

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