Re: Multiple lead-acid charging

eromlignod wrote:
> On Jun 19, 1:36 pm, Peter Bennett <pete...@[EMAIL PROTECTED]
> wrote:
>> The current rating of the charger depends primarily on how fast you
>> want to charge the batteries, and on how much energy you have to put
>> back in them, rather than on the total capacity of the batteries.  
>>
>> If you will regularly discharge the battery by 100 AH, and have a
>> couple of days to recharge, then a 10 amp charger will be more than
>> sufficient.  If, on the other hand, you will need to charge it in a
>> couple of hours, you'd need a 100 amp charger.
>>
>> As another poster said, if you have several batteries permanently
>> connected in parallel (or series-parallel, as you might do to get 12
>> volts from a group of 6 volt batteries), you can consider the group as
>> a single large battery.  Personally, for parallelled batteries, I like
>> to have the possibility to disconnect each parallel branch so I can
>> easily test if any branch is losing capacity (or to readily remove a
>> branch if it develops a shorted cell) (I've had a couple of situations
>> where one of two parallelled batteries failed, and did not notice a
>> problem til I tried to use the bad battery alone...)
> 
> 
> This is interesting and a little confusing at the same time.
> 
> The way I understand battery charging, at some point it is usually
> necessary to produce a constant charging voltage (maybe I'm
> misunderstanding).  But as more batteries are connected in parallel,
> more current is required to maintain the same voltage than for one
> battery.  It seems to me you can't control the voltage *and* the
> current at the same time.
> 
> If you want to only supply, say, 20 amps to a battery, you have to
> provide "x" amount of voltage to force this to happen.  If you have
> multiple batteries, this voltage would be different since the total
> impedance would be different.  I suppose a constant-current supply
> deals with this accordingly.
> 
> But if you want to provide exactly 13.5 volts, the batteries will pull
> "y" amount of current, depending on how many batteries are connected
> in shunt.  In fact, I would think that the amount of current would be
> pro****tionate to the number of batteries, just like resistors in
> parallel.  This would have to eventually outstrip the capacity of the
> charger.
> 
> How would a charger deal with this?
> 
A typical charger for *big* batteries will be a 3 stage charger.

Here follows a typical explanation:



Stage 1. (Bulk Charge)
A soft start delay is activated on connection to the battery to prevent 
sparking, after which a controlled constant current of 0.6 Amps is fed 
into the battery to rapidly raise the terminal voltage to 2.41 volts per 
cell. The length of time taken to complete stage 1 will depend on 
capacity of the battery and depth of discharge.


Stage 2. (Controlled Over-voltage Equalising)
The terminal voltage is maintained at a constant potential of 2.41 volts 
per cell during which time the charge current will start to decay 
exponentially until it reaches a preset level of approximately 10% of 
the chargers maximum current rating. The length of time from start of 
bulk charge stage 1 to this point is monitored (T1). The terminal 
voltage is then maintained at 2.41 volts per cell for a further time 
period of
T1 / 2.


Stage 3. (Standby Mode)
At the end of the timed period the terminal voltage is reduced to 2.28 
volts per cell which is maintained until either the battery is 
disconnected or discharged sufficiently to switch back to stage 1. State 
of charge is indicated by static red LED during stage 1 and stage 2, and 
by static green LED for stage 3 (Charge Complete)



Note that this is from the description of a charger intended for 
relatively small batteries. The 0.6A in stage 1 would take a very, very 
long time to charge a big battery. For many intelligent chargers, this 
very, very long time would be flagged as a fault condition - hence it is 
im****tant to get a charger rated for the AH of the batteries that are 
being charged.

Note that the above description relates to one particular battery 
technology. Different cell compositions require different charging 
profiles. Hence it is im****tant to match the charger to the batteries.


Note also that your suggested "charger", IIRC, is not designed as a 
charger but as a constant voltage supply. These really won't do at all 
as chargers - the current demanded by a flat battery is likely to cause 
the charger to trip out on overload. The voltage output when connected 
to a fully charged battery, if high enough to fast charge it, is going 
to destroy the battery if left connected and powered up.

Charging batteries has to be done with a charger designed for the type 
and size of batteries being charged. Doing anything else is either not 
going to charge the batteries fully or greatly reduce the battery life 
expectancy. Or both.

--
Sue