On Fri, Nov 18, 2005 at 11:14:14AM +0000, Wayne Stallwood wrote:
On Thu, 2005-11-17 at 09:01 +0000, Chris Green wrote:
A bigger battery won't take more charging current, it'll just take longer to charge. The internal resistance of just about *any* lead acid battery will be negligable compared with that of the charging circuitry, the charging circuitry will set the current at any given voltage and will not be affected by the (negligable) internal resistance of the battery.
I think you may be confusing the charging of lead acid cells with dry cells.
Ideally lead acid cells should be charged with constant voltage not constant current as with most dry cells (actually is not as clear cut as that as some of the more modern dry cell technology requires a sort of constant current/constant voltage hybrid charger that switches modes at different points in the charge)
The 'constant voltage' is only an indication of how the charger controls the charging current. Unless the charger has a *ridiculously* low internal resistance the voltage will be dictated by the state of charge of the battery and little else.
I agree that lead acid battery chargers are nominally 'constant voltage' as they control their charging current according to the voltage being output and detect a fully charged battery by the voltage. NiCd and NiMh chargers are (basically, but with caveats) constant current devices.
My point is that a badly designed battery charger for a small capacity lead acid pack could feasibly become damaged by operating beyond it's rated output duty cycle by being connected to a larger capacity pack.
I doubt it, the voltage across the battery will be dictated by the state of charge of the battery. It's the *voltage* of the battery while charging that the charger uses to detect how near fully charged the battery is. A 50% charged 100Ah battery will show the same voltage as a 50% charged 10Ah battery (some assumptions of course, similar battery type and construction, similar state of decay, etc.). The charger will push the same current into both, it's just that the voltage of the lower capacity one will rise faster as it gets charged.
A lot of the better UPS's have smart charging circuitry that cycles the battery (not a deep cycle, just a gentle charge/discharge) and monitors the battery condition based on results from this cycle. I could imagine it getting a little confused if a larger (or otherwise different) pack was connected.
Maybe, but the example in question was (as I understand it) a cheap and simple wall wart type charger I think.