38.9v on First Charge w/ New Batteries

[yurtle]

Originally Posted by JohnnieB

After the surface charge dissipates, cart batteries self-discharge at about 1% SoC (State of Charge) per day, which is only about 40 millivolts (0.040V), so the difference between letting the batteries rest 12 hours and 24 hours isn't much.

Wouldn't 1% of SOC be 400 mv?

[JohnnieB]

Originally Posted by yurtle

Wouldn't 1% of SOC be 400 mv?

It depends on where the 1% is at, but 100% SoC for a 36V pack of Trojan batteries is 38.20V and 99% Soc is 38.16V.

A 400mv drop (38.2V down to 37.8V) would be an 11.5% drop in SoC. (100% down to 88.5%)

Using interpolation of the data Trojan Battery publishes, between 100% and 90% a 1% drop averages 0.019VPC, between 90% and 80% it averages 0.020VPC and between 80% and 70% it averages 0.021VPC.

[yurtle]

Gotcha. 1% drop in SOC is different than 1% drop in voltage.

[Knothead]

Yurtle, I thought you'd figured out by now not to argue with JB!

[yurtle]

Arguing, and questioning for learning are two different things.

If I understand you now, starting off at 100% SOC, you'll drop to around 90% SOC in 10 days.

[yurtle]

With my 48 volt pack, .5 volts reduction in voltage = 10% reduction in SOC, so that works out to 50 mv. If I hadn't asked, I would still be thinking a pack loses 1% in voltage per day, even though that's never what was being said. I was seeing the words, but reading them incorrectly.

[JohnnieB]

Originally Posted by yurtle

Arguing, and questioning for learning are two different things.

If I understand you now, starting off at 100% SOC, you'll drop to around 90% SOC in 10 days.

I was the only boy and raised with four sisters, two older, two younger, so I lost more than just a few arguments, but I did learn a few things about the way females think and I've reduced it to a simple formula: 2 + 2 = Chartreuse.

You are on the right track, but like everything else about electric carts, nothing about batteries is an absolute and everything is a compromise.

The 1% SoC per day self discharge rate is approximate. Some batteries self-discharge faster, some slower and some carts have more parasitic current flow than others. So you more or less have to measure it to find out what it is for your cart.

[cooose]

My first reading of 38.9 was within about 30 minutes of the charger kicking off on the first charge. This is why I was expecting the reading to be a bit higher. I drove it about 5 miles yesterday then hooked it back up and monitored how long the second charge was. The charger stopped after ~ 4hr and again pack was measuring below 40. I thought I remembered reading somewhere that I should expect 8-10 hours or more for initial charging. Could the timer or sensor in the powerwise be faulty causing the charger to stop prematurely? Any way to test this?

[yurtle]

Did you ever get a reading 12 hours after charging completed?

[cooose]

Yes the +12hr reading was 38.5, 5 batteries at 6.4, 1 battery at 6.5.