Should deep cycle gel batteries be fully charged on a regular basis?

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simara asked Jan 28, '23

Should deep cycle gel batteries be fully charged on a regular basis?

I have suffered a premature failure of my deep cycle gel batteries. 5 years, 320 discharge cycles to not less than 60%. A small number of discharges to 40% and one accidental complete discharge. The batteries have always been on intelligent charger set to the recommended settings. Loads are only infrequently more than C/100. The batteries have essentially all gone to a high internal resistance state and drop below 9.5V at an indicated 18% discharge. Fully recharging gives me a calculated CEF of 95% which might give some clue to the failure “chemistry”. This is a boat installation where the batteries are rarely fully charged. Solar + wind is topping them up from 60-80% pretty much every couple of days day. However they are only occasionally fully charged when on shore power. Does anyone know if my premature failure is symptomatic of this usage profile and if so, what replacement technology might be best (Lithium would be out of the question price wise).

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Answer 1 · 2023-01-28T15:46:10Z

wkirby answered · Jan 28, 2023 at 03:46 PM

Lead acid based batteries do regularly require a full charge in order to prolong their lifetime.
I know that you said that Lithium based batteries would be beyond your budget, but they are much less sensitive to not receiving a full charge very often. They like a full charge periodically to balance the cells.

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kevgermany ♦♦ commented · Jan 28, 2023 at 07:10 PM

@Simara

You could take a look at lead carbon. They've better resistance to being undercharged like this. Might be worth getting a small generator.

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simara commented · Jan 29, 2023 at 06:29 AM

Its not so much the cost of the LiFePO batteries themselves but the additional complexity in the system. For example I have a high power alternator connected through a low loss charge splitter that charges both the service and starter banks. Mixing LA and Li batteries would mean a dc-dc converter/charger (I guess?) to charge the LA starter. Moreover, my thrusters run from the service bank and I can not run the thrusters from Li so I would need additional LA banks and the associated charging systems. The internal resistance of Li is too low for the DC motors in the thrusters. When they spin down they act as a dc generator which momentarily pushes current back in to the batteries. With a low resistance battery this current can damage the motor windings. I learned my lesson on this as I destroyed an electric winch motor when I moved from standard LA to gel. Even this drop in internal resistance was too much!! I guess that a ballast resistor or big!! diode might solve this issue. I believe that newer winch and thruster motors have “intelligent“ protection to stop this issue.

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