Re: (ET) Happiness...

["David Roden (Akron OH USA)" <etpost drmm net>]

On 9 Feb 2011 at 18:14, Max Hall wrote:

> do I diminish the service life of my batts at all to charge them at
> c/20 or c/16? 

Probably not, with a couple of "buts" related to this specific situation.

First, you should know that some batteries, such as Genesis AGMs, 
absolutely 
require high current charging or they quickly lose capacity.  Hawker used 
to 
recommend C10 / 3, that is, one-third of the 10 hour amp-hour capacity 
expressed in amps.

As it turns out, they need this for only a short time - even as little as 
a 
few minutes - at the start of the charge cycle.  The rest of the charge 
can 
be at a more typical level, say C/10.

I'm not an electrochemist, so I don't really understand why this is, but 
it's a verified phenomenon.

Beyond this, IIRC it's a rule of thumb that you should charge a battery at 
a 
peak initial current of around C/10 - that is, one-tenth of its 20 hour 
amp-
hour capacity expressed in amps.  Thus the optimum initial charging 
current 
for a 220 amp hour (at 20 hours) battery is around 22 amps.  

BTW, to clarify the nomenclature above.

C or C20 is the battery's 20 hour amp hour capacity.  For a typical old 
school golf car battery, this is 220 amp hours.

Similarly, C5 would be the 5 hour amp hour capacity, always less than the 
C20 capacity.

10C would be a current that is 10 * the C20 capacity expressed in amps.  
For 
example, 10C for a 220 amp hour battery would be 2200 amps.

C/10 would be a current that is one-tenth the C20 capacity expressed in 
amps.   For example, C/10 for a 220 amp hour battery would be 22 amps.

Be careful not to confuse C/10 with C10!

OK, now back to the question.  Flooded golf car batteries are pretty 
tolerant.  You could probably go down to around 6-8 amps with no major 
problems.  In general, I'd say that you're OK as long as it takes less 
than 
24 hours to reach 80% full. 

So I think that half current would be OK.  But here's the catch to your 
plan.  Your strings almost certainly won't divide the charging current 
equally, since one of them is mostly worn out.  

This means that you really should have separate chargers for those strings.

One interesting and economical way of doing this might be to split the GE 
charger's output.  Remove the single diodes, and replace each with a 
suitably sized bridge rectifier.  Now you have two separate-but-equal 36 
volt chargers, each capable of half the power, and each string can have 
its 
own.

Er, not CAN, but rather MUST.  You will have to separate the two strings 
when charging.  They can't be charged in parallel with this scheme.