On Fri, 2005-11-18 at 12:44 +0000, Chris Green wrote:
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.
I think perhaps we are talking about the same thing but using different terminology...hence the confusion.
By my understanding.
Most lead acid chargers charge at a constant voltage, a car battery charging circuit will for example charge at a constant 13.8v (or whatever, depending on the delivery capacity of the charging circuit) (12v being the nominal voltage of a car battery) This voltage is output regardless of battery charge condition...naturally the charge current then goes down as the charge progresses (and therefore as you say the battery voltage goes up) Because the output voltage is higher than the nominal voltage of the battery, once charged the offset is calculated to equal a acceptable trickle charge (given the internal resistance of the average battery)
This is one of the (many) reasons why we now use alternators in cars rather than dynamos, it is hard to build a constant voltage charge around a dynamo supply unless the dynamo is running at a constant speed. This is also why a current meter in a modern car is far more useful than a voltage meter.
Most smart charging circuits don't actually measure the battery voltage because this is difficult to do during the charge process. They simply measure charge current and when it hits a low trigger the charge is considered complete. This is important in a fast charger because in order to achieve a fast charge the output voltage may be too high to drop to a acceptably low trickle charge (stick 15 volts into a 12 volt lead acid indefinitely and you will just boil off the acid, which apart from not being too good for the battery, is pretty dangerous in a confined space)
Dumb (quick and dirty) NiCd (and NiMh) chargers operate in the opposite way, you set a hard current limit (for trickle this is usually capacity/10, at least that is what I have set for the charging circuit I have just built for my portable mp3 streamer project). This current limit is held regardless of the internal resistance or voltage of the pack connected, although naturally if the pack voltage gets close to the charge voltage this may drop. You can determine the charge state by looking at the voltage across the pack (even when on charge) with this circuit because your Constant Current circuit will be adjusting the charge voltage to achieve the desired charge current, therefore the higher the charge voltage the nearer you are to full charge.
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.
Sorry I wasn't very clear, my point was that I bet a lot of the cheaper wall wart chargers are duty cycle limited. Given a constant voltage charge the charger will be delivering the same current at the start of a charge for a 10Ah pack as a 100Ah pack but it will be delivering that maximum current for 10 times as long, given manufactures habits of designing everything down to the minimum this could quite likely result in meltdown.
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.
Again that was my fault, I went of on a tangent about different charge methods (from the original subject of connecting a random 12 lead acid battery to a UPS in place of the original pack) and then snapped back on topic without telling anyone. I was in Holiday mode...sorry.