Quote:
Originally Posted by Joe K
The result seems to have sufficient battery capacity for 8 hours of sump-pumping (well above what one would expect) BUT the high end charger "boils off" his electrolyte over a period of 5 or 6 months requiring a top-off. (automated, the system gives alarm on low electrolyte.)
I suggested adding a timer to turn the charger on perhaps 20 minutes a day.
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What you're describing, and what others have alluded to, is a function called
float voltage. Good chargers need to reduce the voltage being sent to the battery when it's fully charged. The proper long-term voltage varies by both battery type and ambient temperature. If the voltage is too high, the battery dries out. This is a common problem with UPS (uninterruptible power supply) batteries. Some vendors' batteries wear out after two years because their float voltage doesn't adjust for how warm those batteries can get.
Many "smart" charging solutions go through a set of programmed steps. For example, the Mercedes charger does this:
- Charge at 3.6A until detected battery voltage reaches 14.4V
- Gradually reduce amperage to 0.4A
- At 0.4A, shut off and monitor battery voltage
- When battery voltage drops to 12.7V, repeat cycle
If your sump pump battery isn't playing by the charger's rules (for example, if it never gets to 14.4V or if it drops quickly to 12.7V), then the charger will spend too much time in full-charge mode and it'll boil off the electrolyte.
Bottom line: Smart chargers can be good or bad for long-term charging depending on how the battery being charged reacts to the charger's float voltage or intermittent charging cycle. You should read your charger's manual to understand what it's actually doing and make sure your battery is compatible.