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[DPRG] Remedial Circuitry

Subject: [DPRG] Remedial Circuitry
From: Ralph Tenny rften at swbell.net
Date: Fri Dec 5 12:47:01 CST 2003

Earlier, I replied to the battery charging issue, but failed to send to
the whole group. What has been discussed so far is related to lead-acid
and gel cells. I noted that NiCads should be charged by a current source
sufficient compliance to handle the largest stack you expect. This
that each cell receives the same charge.

-----Original Message-----
>From: dprglist-admin at dprg.org [mailto:dprglist-admin at dprg.org] On Behalf
Of Karim Virani
Sent: Tuesday, November 25, 2003 2:43 PM
To: dprglist at dprg.org
Subject: [DPRG] Remedial Circuitry


I'm trying to charge the batteries on my robot and am trying to come up
with a circuit that will change the configuration of the batteries to
allow a standard commercial battery charger to be used.  I have 3 7ah
12v SLAs, A, B & C.  A & B are normally wired in series to create 24v
for the drive motors.  Battery C is used to power the PC-based brain, a
couple of OOPICs and various sensors.  The idea is to dynamically
re-wire the batteries all in parallel to allow charging when an
off-the-shelf 12v charger is connected.

The circuit I came up with is posted here:

It uses 2 relays (DPST-NO and SPDT) - the empty circles in the diagram
represent the relay coils.  Right now I'm going with digikey part nos.
Z963-ND and Z848-ND.  It doesn't matter that the drive motor voltage
drops to 12v during charging, but the charger does need to continue
supplying current to both motor and control circuits while charging the
batteries.  I haven't measured the amperage that the control circuits
consume - but it's probably on the order of 2 amps.

Assuming this circuit is somewhat reasonable, I have a few questions.
When considering your feedback, bear in mind I've no electronics
training and this is the first circuit I've drawn.  

1.  Where did I screw-up?

2.  I'd like to charge at pretty high rates - I seem to remember that
you can charge SLAs at up to 1C for hobby use.  Does that sound right?
Does that mean I could charge at up to 21A for 3 7ah batteries in
parallel?  My real goal would probably be a 5A charge rate.

3.  Given the above, can anyone suggest a part number for the diode -
assuming I used it correctly?

4.  Batteries A & B should be at similar discharge levels, but C could
be quite different.  Can you parallel-charge SLAs like this?  Will the
current automatically flow faster to the batteries that need it most?

5.  Where would be the best place in the circuit to include battery
level monitoring?

6.  What kind of protection issues do I need to consider - where would
be good spots to insert fuses, etc.  I do understand the difference
between nominal voltages and real voltages in SLAs.  It seems most
chargers will put out 14.7v max when charging and the control power
supply can handle that.

Thanks for any advice,

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