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 <[email protected]>
To: Electric Vehicle Discussion List <[email protected]>
Cc: Lee Hart <[email protected]>
Sent: Tuesday, September 12, 2017 7:36 PM
Subject: Re: [EVDL] Slow due to 96V pack?
From: paul dove <[email protected]>
> 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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