Re: (ET) Battery charger question

["David Roden" <etpost drmm net>]

On 11 Oct 2021 at 20:07, Rob Brockway via Elec-trak wrote:

> For a t105 size battery what do you think is the maximum safe discharge
> amps for 30 seconds for example? 

I've seen golf car batteries produce over 700 amps for several seconds at 
a 
time, and I've heard of people getting 1000 amps out of them.

But you asked what's the SAFE discharge current.  I don't think I'm 
knowledgeable enough to give you a solid answer, but I'll take a somewhat 
conservative guess and say that you could probably draw 500 amps for 30 
seconds without much hazard.

I say that because golf car batteries are designed to be reasonably happy 
delivering 75 amps all day long (well, until they're flat :-), with short 
term peaks of around 300-400 amps.  That's what golf cars generally ask 
for.  

Now if you have a battery with one or more weak cells, especially below 
50% 
state of charge (SOC), that's a different situation.  Now you have the 
risk 
that the weak cell will go flat before the others do.  When you ask for 
big-
arse current, the other cells say, "sure, let's go!"  But to get that 
current to the load, they have to ram it through the flat cell.  

The current actually tries to charge that cell *in reverse* - and FAST.  
The result can be what a friend of mine used to call a "Trojan teakettle." 
 
The cell can even explode from the steam pressure.  When that happens to 
you, a good day is not in the cards.

As a rule, all other things being equal, blah, blah, blah, the larger and 
heavier the battery, the more current it can provide.  

Ask a forklift battery for 1000 amps and it says, "All in a day's work." 

On the other hand, ask a 12 volt group 24 or 27 marine battery for a few 
hundred amps routinely and it's likely to turn toes up in some dozens of 
cycles.  Marine batteries are designed (there's that word again) to power 
trolling motors and lights, so they're happy at 25 amps - 1/3 of a golf 
car 
battery's nominal design current.

AGM batteries are a special case.  I've forgotten the physics of why, if I 
ever knew, but even small ones can often - again, if designed for it - 
squirt out currents in the thousands of amps without blinking an eye.  

There are also lithium batteries that can produce stupendous amounts of 
current short-term - and those that can't.  Again, it's a matter of the 
compromises the engineers have made.  


David Roden - Akron, Ohio, USA

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