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Discussion Starter #1 (Edited)
From all my searching around I have been given a figure of 1.75V/cell as the lowest you can safely go with lead acid, but I am not sure what this means. Does this mean resting voltage, like when I let off the throttle, or does it mean voltage under load?

My pack is 72 volts worth of used dual-purpose commercial truck batteries. When fully charged, the lowest they will sag to under max power output through my Alltrax 7245 is about 65 volts (1.8V/cell). However, after the batteries are drained a bit it takes nothing to get them below 60 volts while accelerating even a little. At one point yesterday I saw 49V (1.4V/cell) on the meter when I was trying to get er home, foot buried, and freaked out because I thought I was lunching my pack. When I let off the throttle completely, system voltage still jumped to 71V.

Is it okay to drag them so far down under load? It scares me.
 

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You really should try to stay above 1.5 vpc under load. Lower than that seems to kill available range and slow voltage recovery when you let up on the throttle. When you release the throttle the pack should bounce back over the nominal pack voltage almost as fast as you can look at the volt meter. I've found some golf cart batteries have better voltage recovery during a drive if you don't let them under 1.75 vpc.

1.95 vpc with no load for 4 hours is 0% SOC. 100% SOC can vary depending on a number of factors but is around 2.15 vpc for a battery that has seen no charging current for at least 8 hours.
 

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Discussion Starter #3
Thanks. That makes me feel much better.
When I let off, the voltage does jump back up instantly. 1.5vpc = 54V for me. I could have stayed above that if I would have been concentrating harder.
I pinpointed a very weak battery in my pack today (after load tested each one) and replaced it with [hopefully] a better one, so hopefully I don't have to think about this for a while anyway!
 

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Every battery is a little different. For mine lower than 1.75 VPC under load is bad, and lower than 1.5 VPC is really bad.

You really can't measure SOC just on resting voltage. I've taken batteries to 0% useful SOC (there's still lots left if you're running a light bulb, but not an EV) and they'll still come back to nearly 2.1 VPC.

One (or a few) bad cells can easily drag the entire pack down. Some batteries die 'nicely' by sagging to 0, then go into reversal down to negative nominal, but only affect the pack by subtracting linearly.

Some will die by turning into an open circuit, where the entire pack's apparent voltage sags exponentially, leaving the car shuddering and you searching for a safe spot to stop.

Just like with lithium, I think an AH counter and/or half pack monitoring is a good idea to know when one cell/battery is bad or the whole pack is tired.
 

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The best way to measure is with AH counter, voltmeter under load, and experience. By the time you get serious sag (below 1.75 vpc ~60-70% DOD on mine) you've used too much, so I think you have to learn how far is too far using the DVM, then use the AH counter to not go that far.
 

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I've pounded both Optimas and golf cart lead. Both may sag below 1.75 vpc at reasonable loads. Golf cart lead at 60% SOC may drop below that at 400 amps, sometimes less (especially 8 volt batteries.) At 400 amps an Optima will drop down there at about 30% SOC. And then there is the effects of cold -- way more sag under load.

The whole concept of SOC is a bit fuzzy with lead. At EV loads a wet type deep cycle battery can deliver about half its 20 amp hour rated capacity. The battery isn't dead at that point, it will simply sag excessively if you try to go farther (and get hot inside and reduce life.) The capacity of lead changes a lot more based on temperature and the age of the cells too.

An amp hour counter is the best way to determine SOC, but it needs to be adjusted to the actual available amp hours and should be temperature compensated. The old E-meter was a popular. I found it worked better if Peukert's exponent was set to 1.0 and an intelligent estimate of available capacity was used, I think about 100 amp hours for the golf cart batteries I used. That was roughly the available capacity at 200 amps, so even then it was sagging more under peak loads after racking up some miles. I don't miss them.
 
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