Lee,
You said:Here's my guess: A 6v golf cart battery has an internal resistance of 
about 5 milliohms (0.005 ohms). A 96v pack has 16 of them; so the resistance is 
0.005 x 16 = 0.08 ohms. Let's say the Zilla limits voltage to 72v (i.e. it 
won't pull the pack below 72v). Then the most current you can get is I = V/R = 
(96v-72v) / 0.08 ohms) = 300 amps.
**It is either going to be 96v/0.08, 72v/0.08, or somewhere in between. The 
voltage won't be 24/0.08 at any time.So, somewhere between 1200 and 900 amps.

In practice, the resistance of the pack is a bit higher due to the wire and 
connectors. So a 200a max current is probably the most you can draw from the 
golf cart batteries before their voltage under load falls to 72v.
**obviously something else is going on since you don't see 1200 Amps.

Now, if you had a different kind of batteries with less internal resistance, or 
a controller that would cheerfully pull the battery voltage even lower, you'd 
have a lot more current, and peppier performance.

For example, my old ComutaVan had a 72v pack of golf cart batteries, and a 
contactor controller. It would cheerfully pull 1000a from the batteries, and 
spin the tires if you "floored it" from a dead stop. I tried a Curtis 1221 400a 
controller; and it was *worse* than the contactor controller for accelleration, 
because it never came anywhere near 400a due to voltage sag.
**Exactly, the controller is influencing the current which is what I said. I 
ould be surprised if you could get a sustained 900 amps out of a lead battery 
but I will have to take your word since I've never tried it.


      From: Lee Hart via EV <ev@lists.evdl.org>
 To: Electric Vehicle Discussion List <ev@lists.evdl.org> 
Cc: Lee Hart <leeah...@earthlink.net>
 Sent: Tuesday, September 12, 2017 7:36 PM
 Subject: Re: [EVDL] Slow due to 96V pack?
   
From: paul dove <dov...@bellsouth.net>
> Current out of a battery is not calculated based on internal resistance.

Actually, it is. The Equivalent Series Resistance (ESR) of a battery is not a 
theoretical number. It is found empirically by measuring the voltage drop 
caused by a high-current load. The voltage of a 6v golf cart battery really 
does drop by about 2.5 volts with a 500 amp load. In other words, it acts like 
an ideal 6v battery with a 0.005 ohm resistor in series. Thus, it's a 
reasonable way to estimate what you are likely to get.

> you won't get 100% efficiency out a controller.

No, of course not. But the efficiency is pretty high, and can usually be 
ignored for back-of-the-envelope estimates. That's all I was trying to do with 
my estimate.

>The battery industry uses a 100-hour rate as an index to compare batteries of 
>different types and sizes.

They do when they are trying to exaggerate the capacity of their battery. For 
EV use, the 100-hour rate is useless. Even the more common 20-hour rate is 
nowhere near the rate at which the battery will be used. What you want to look 
at is the 1-hour or Reserve Capacity rates, which are what you will get when 
discharged at high currents. For a golf cart battery, the reserve capacity is 
measured at 75 amps, and the 1-hour rate at about 100 amps.

But, I don't think these capacity numbers are what is limiting the maximum 
current in this case. My guess is that these golf cart batteries can only 
deliver 200a before their voltage sags to 72v and the controller starts 
limiting current.


--
Excellence does not require perfection. -- Henry James
--
Lee A. Hart http://www.sunrise-ev.com
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