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Old 12-13-2023, 12:15 AM   #1
rockfordpi
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Join Date: May 2016
Location: Central Alabama
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Default Why a 105AH Lithium >= 170AH Lead Acid explained

I get this question a lot and have seen several posts related to this so I thought I would share this. My apologies if this has already been explained better somewhere else.

This is a question that use to bug me a lot until I finally figured it out the help of the calculator below. My battery reps were unable to adequately explain this to me (I am not sure they actually knew, Gosh I miss Johnnie B).

As it relates to a GC, a 170AH T-875 is really NOT really a 170AH battery.

For the sake of comparison, batteries as a standard are rated at their 20 Hour Rate which means that, under optimal conditions, you can get 170AH of energy from a T-875 IF you drain it over 20 hours, which would be 8.5 amps/hour (170/20).

Nobody drains a GC battery over 20 hours, I doubt 8.5 amps would even move the cart after it was rolling.

A GC pulls much more amps and (roughly) is drained over a 2 hour or less period. Peukerts law implies the amount of energy you can get from a constant drain on a lead acid battery is inversely related the rate at which you draw the amps. The Higher the amp load, the less energy you can get from the battery. This is indicated in the T-875 specs:

100 hour rate: 189ah
20 Hour rate: 170ah
10 Hour rate 155ah
5 Hour rate 145ah
2 Hour rate 109 ah (calculated below, not given by Trojan)

A good rough average amp draw for a 48v Stock Cart is 56 amps. At 56 amps a T-875 will drain in roughly 2 (1.95) hours. So, the 2 Hour rate on a T-875 is 109AH (links to calculations at end). So, as it relates to a STOCK Golf cart, a T-875 is a 109AH battery (even less than 109AH if it is used on a cart with larger tires and/or High amp controller).

Now, Peukerts law does not apply the same way to Lithium batteries. A 105AH lithium will give you one amp for 105 Hours OR 105 amps for 1 hour. The rate of discharge does not change the amount of energy you can get from a Lithium like it does a lead acid Batt, you get the full 105AH (that is why lithium does not give a 5 hour rate or a 20 hour rate). While these ratings are not published by the manufactures, rates that compare to the T-875 are:

100 hour rate: 105ah
20 Hour rate: 105ah
10 Hour rate 105ah
5 Hour rate 105ah
2 Hour rate 105ah

Now, couple this with the weight savings and the fact that Lithium does not slow down when it gets low and you can get the full capacity from the battery without shortening its life, a 105AH Lithium has MORE EFFECTIVE capacity than a 170AH T-875 pack.

For those nerdy people like me, there is a very good calculator to verify this at http://www.evdl.org/uve/battery.html.
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Old 12-13-2023, 05:00 AM   #2
Pat911
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Default Re: Why a 105AH Lithium >= 170AH Lead Acid explained

That’s a very good explanation, but it’s even better than that. Lead acid batteries can only be taken down to 50% SOC if you want any reasonable service life out of them. So, in the example above, that 170Ah battery really only has 55Ah useable at the 2 hour rate, not 109Ah. The 105Ah lithium has the whole 105Ah available if you need it and it will still meet the manufacturer’s claimed discharge cycle count.

My rule of thumb is that you need about 1/3 the capacity when upgrading lead acid to lithium to get the same range. A typical 74Ah 48v lithium battery would be very close to 225Ah LA, and a 105Ah lithium……315Ah LA.
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