Battery question

stellix

I installed my Plum quick Bandit and it seems that my run time is a much shorter now. I didn't notice much if any speed or torque increase. After a 3 mile trip today on a full charge I notice a huge decrease in speed going up a hill and my SOC meter was showing less than 50%. The funny thing is that with the SOC meter on less than 50% my pack voltage was 50.3volts. Could I have a bad battery that loses power under a load but still shows good voltage when tested?

BGW

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gornoman

Yes, that is very common. Easy to test also.

stellix

What's the best way to test them? The batteries are only a little over a year old, if I find the bad one do you think it would be ok just to replace that one or should I replace them all? I checked them while the SOC meter said 40% and they each showed 6.3 volts and the pack voltage was 50.3. (eight 6 volt batteries). Going by the charts that should still be a good charge but my SOC meter says 40% and the cart feels weak while going on a mild uphill road.

gornoman

Set your multimeter to 20vdc, attach the probes to each single battery, and drive the same course each time. If there is a weak battery it will show up in this test. Be sure the test includes a nice hill if one is nearby. They sell multimeter probes with alligator clips built in that makes this test much easier.

kellybell

Gornoman, What is the air-speed velocity of an unladen swallow??

stellix

I just finished testing each battery. I hooked my meter to each one and made the same uphill run about 300 yards. Before I started 7 batteries were 6.3 volts and one was 6.3 . During the test 7 of them would drop down to around. 5.5 under a load and one dropped to 4.9 volts. They all seemed to recover back to 6.3 and 6.2. Is the one dropping to 4.9 under a load bad? How much voltage drop is acceptable? SOC meter seems to be dropping quicker than it should.

JohnnieB

When doing individual battery testing, you need a voltmeter that measures down to 0.01 volt because the difference of 0.1V is a difference of about 15% SoC at the 6V battery level. (IE: 6.34V is 95% SoC and 6.25V is 80% SoC, but both round off to 6.3V.)

Sounds like you have a weak battery, which will limit the amp output other seven batteries.
If possible, replace it with a good used one the same age, make and model as the others.
If you can't find a used one, will work, but the new one will soon have the same aging characteristics as the older ones, so when it is some to replace the pack, replace all of them at the same time.

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The amount battery voltage drops under load depends of how many amps are being drawn and the storage capacity of the battery and its SoC and the temperature and some other things, but they are connected in series and should all drop about the same amount if the pack started out being balanced.

For routine operation, staying above 1.75VPC (Volts per Cell) under load would be a reasonable goal, which is 42.0V for a 24 cell (48V) battery pack.

For emergency situations, dropping to 1.5VPC (36.0V for a 48V pack) is acceptable, but going below that will cause irreversible physical damage to the battery plates.

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Sounds like you have a LED Bar-Graph type battery meter.
Do yourself and your batteries a favor and replace it with a DVM type.

sportcoupe

Found your answer:
http://style.org/unladenswallow/

stellix

I have a DVM but I havent installed it yet. I have a Curtis bar type meter in it now , I just don't trust it. I'm thinking about replacing all the batteries with 6 new 8 volts. I know y'all said my run time will be shorter but it seems short now. How far do you think I could go on 6 Trojan 875's? I'm running 20" tires an Alltrax DCX 400 and a Plum Quick Bandit on mostly flat ground

JohnnieB

It won't go as far as it would with the US 1800's if they were in good condition, but it doesn't sound like they are in very good condition, so new T-875's will most likely go further, perhaps a lot further.

However, since you are staying at 48V, run-time is directly proportional the the battery's AH rating and while there is some overlap between the AH ratings of the readily available 6V and 8V golf cart batteries, 8x6V battery packs typically have more run-time than 6x8V battery packs.

Is the reason for going from 8x6V to 6x8V, space or money?

If space isn't an issue, you can probably find eight economy grade 6V batteries for not much more than the cost of six T-875's and very likely less than a set of the highest AH rated 8V batteries.

Bottom line: A 48V battery pack consisting of eight store brand 6V golf cart batteries with a AH rating above 208AH will have more run-time than a 48V battery pack made from the most expensive 8V golf cart batteries on the market.