Series Connected FLA's not Charging Uniformally

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Papa   1 kW

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Series Connected FLA's not Charging Uniformally

Post by Papa » Sep 01 2018 4:16pm

I have 10 100w PV's charging four, series connected 12v, 114Ah batteries. I'm using a Victron 150/35 controller set at 48v. The batteries are less than 4 months old and ALL purchased NEW at the same time (two even have sequential serial #'s). Problem is.... the center two batteries (#2 and #3) are charging at approx. .3V higher than the end two batteries (#1 and #4). Swapping the batteries individual position within the string, doesn't change the charge voltages - it's always the center two batteries being over charged at .3V to .4V higher than the end batteries. At rest, all measure 12.85V and all SG's are nearly identical. What's causing this?

edit: All cabling between batteries is 4 ga.
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amberwolf   100 GW

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Re: Series Connected FLA's not Charging Uniformally

Post by amberwolf » Sep 01 2018 10:48pm

Does the charge controller connect only to the two ends? Or does it connect to the inter-serial battery connections? (to all the +/- tie in points in between batteries)

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Re: Series Connected FLA's not Charging Uniformally

Post by billvon » Sep 02 2018 12:14am

Papa wrote:
Sep 01 2018 4:16pm
I have 10 100w PV's charging four, series connected 12v, 114Ah batteries. I'm using a Victron 150/35 controller set at 48v. The batteries are less than 4 months old and ALL purchased NEW at the same time (two even have sequential serial #'s). Problem is.... the center two batteries (#2 and #3) are charging at approx. .3V higher than the end two batteries (#1 and #4). Swapping the batteries individual position within the string, doesn't change the charge voltages - it's always the center two batteries being over charged at .3V to .4V higher than the end batteries. At rest, all measure 12.85V and all SG's are nearly identical. What's causing this?
For FLA's voltage isn't the most important parameter - it's specific gravity. If all SG's are close (and it sounds like they are) you are good. If they're not, do an equalize charge.
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gogo   10 MW

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Re: Series Connected FLA's not Charging Uniformally

Post by gogo » Sep 02 2018 9:10am

Temperature?
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Papa   1 kW

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Re: Series Connected FLA's not Charging Uniformally

Post by Papa » Sep 04 2018 4:29pm

Sorry for the late update - way too much on my plate now:( .

Amber, Batteries are connected and charged serially with only a single 50v connection off the end batteries. I would much prefer a li-ion bank, but that purchase needs to wait until next summer.

Battery & controller temperature is 75f.

SG is now slightly lower at about 1.260 - some cells slightly lower, some slightly higher.

But again, why would individual battery readings be different (as high as .4V), while charging... but less than .05V at rest? Measured w/Fluke 87-5. Thanks Guys.
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Re: Series Connected FLA's not Charging Uniformally

Post by amberwolf » Sep 04 2018 5:22pm

Papa wrote:
Sep 04 2018 4:29pm


But again, why would individual battery readings be different (as high as .4V), while charging... but less than .05V at rest?
Normally I'd say different internal resistance (those with higher resistance develop a higher voltage while charging...and sag more during discharge).

But since you've swapped which batteries are in different positions, and the issue follows the position and not the batteries, I have no idea. Doesn't make sense to me, since there's no taps from individual batteries, only the end total + and -.

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gogo   10 MW

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Re: Series Connected FLA's not Charging Uniformally

Post by gogo » Sep 04 2018 6:29pm

Temperature.

The center batteries can't shed heat as well as the end batteries. Warmer batteries will show a higher voltage. When temperatures equalize, so do voltages.

There are charts showing what the charging voltage needs to be according to temperature. Cold batteries need less voltage during charging.
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