Tom,

Good points, those. I just need to be able to charge the thing more or
less conveniently while I figure out a "better way".

OK, in the short term, suppose I substitute a 7.4V zener and put a
blocking diode in the 12V battery arm? That way the 12V 7AH can't
discharge thru the ZD+6V 4AH battery. The 2 batteries would then take
current separately off the external charger. Trouble is the little 6V
fellow would fill up before the bigger 12V one was fully charged.
Maybe I could put a 2PDT switch in the 6V arm and disconnnect it
manually when it had enough?

This is all pretty stop gap - the battery pack has to power a camera
in a remote location and for some weird reason I figured an 18V rail
was needed. Bad idea. What I'll do "sometime" is build a switchmode
supply with an 18V o/p, run the camera & etc off the 12V battery, fire
up the 18V rail from that & I can just have a single 12V charger.
That's in the long term, though ...

Ian.

Tom Messenger wrote:
>
> How to charge lead acid batteries
>
> You need two numbers: charge voltage (assuming you want 100% charge) and
> float voltage (assumes you will use a trickle charge rate to keep the
> batteries "floating" near full charge to prevent self discharge.
>
> For 12V lead acid (either wet or gel cell) batteries, the charge voltage is
> 14.8V and the float voltage is 13.8V. (not counting temperature extremes,
> other unknown factors, etc.)
>
> If you bring up a discharged battery to 13.8V, it will only be about 80%
> charged.  It needs to be brought all the way to 14.8V before allowing to go
> back down to 13.8Volts.
>
> In your circuit, I guess you are charging with a low current source,
> probably to avoid roasting the zener diode. The "6" volt battery at float
> is 6.9V and with the zener voltage is 12.5Volts. As you transition from
> float to charge, the current begins to rise. (Continuous charging at higher
> voltages begins to dry out the electrolyte and seriously shorten the life
> of the battery.)
>
> So it looks like your 6 volt battery and zener are clamping the system
> charge voltage way too low for the 12 volt battery to be happy.  A
> simplistic answer is to size the zener up to around 6.9 volts. And test.
> Maybe you'll get lucky... but due to the unequal capacities of the two
> batteries, you will have to charge the 6 volt battery longer than it
> 'wants' to get the 12 volt battery charged.
>
> Best regards,
> Tom Messenger

> Er, sorry. Of course, what I *meant* to say was size the zener up to around
> 7.4 volts, *not* 6.9volts. This will allow the 12 volt battery to come up
> high enough to get a full charge.  Your charging system will need to
> provide enough  current to satisfy the total of both batterie's currents
> added up.

>
>

--
http://www.piclist.com hint: PICList Posts must start with ONE topic:
[PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads