Math is Hard-- C Rate ?

BFL13 wrote:
I see now, that "C-Rate" has a more specific meaning to many, and also that graph makers and the rest of us are looking for a short-cut way to express current wrt the AH size of the battery bank. Looks like "C-Rate" has been taken, so we can't use that?

Trojan in their guide uses the form "C20" for the capacity at the 20 hr rate. (5 amps per 100AH)

(for AGMs) "as the current slowly declines. The
absorption phase ends when the current stabilizes at a low value of approximately 0.005 x C20"

We say that AGMs need to be charged at 14.x volts until amps taper to
"0.5 amps per 100 AH of battery bank". Trojan says that is "0.005 x C20", while that other guide uses those CA numbers , so they would say that is "0.005CA"

When my actual battery bank of 200AH settled at accepting 0.2 amp at 14.6v, ( 0.1 amp per 100AH) Trojan would call that 0.001 x C20, 1/10th of that "0.01CA, so their graph does not get low enough for long enough in time I guess to reach True Full.

So what is "C/5" really? Batt U. says C-rate of 0.2C = C/5, the five hour discharge rate. ISTR LFP specs saying they can be charged at 1C or like BB, recommended at 0.5C.

it almost looks like your trying to combine two things that are totaly different to come up with some sort of rule. mind you I am not sure if you are asking a question or trying to reason somthing here either.. but 1C, .5C and such have nothing realy to do with C20, C5 or that. well they kinda do but are not relatable. both deal with the internal resistance of the batgtery and how it can change as the charg level and temp of a battery change. (I may be leaving some stuff out but hopefully I am remembering the basics)

the first one 0.5C, 1C and such is an overall measure of the charging curent a battery will accept. it isnt nessasarly for 100ah unless that is the size of the battery it is on. a 1C on a 25AH battery means you can completly charge it from dead using a 25amp charger in one hour. on the same battery a .5C means you could charge it with a 12.5amp charger in 2 hours and no faster a 2C means you could use a 50amp charger and charg it in 1/2 an hour. so that directly relates to the rate of charge for the totaly capacity of the battery. if you had a 2000AH bank of li with a 1C rate you could charge it in 1 hour with a 2000amp charger if you had or could afford one. most of the time this number wont mean anything to the average camper as your camper charger wont put out enough amps to reach the charging limit during bulk. , unless your running a single SiO2 as we have recently found out.

now the other number C20 is basicly how long the battery will last at a constant output over a period of time so C20 is discharging the total battery over a 20 hour period at a constant draw. C5 is over a 5 hour period, C10 is over a 10 hour and I think I have seen C40 also. this number has all to do with the Peukert effect and as far as for us is a convenient way to compar capacitys of batteries on an even playing field . We use the C20 capacity to compare one battery to another. if this info wasnt published then battery makers could pick the best rate publish that as the AH capacity and not tell us which one they used.

so people dont have to look it up Peukert effect says that the capacity of a battery decreases as the rate of draw increases and its a different rate of increase vs. decrease depending on battery chemistry and such. so lets pick 100Ah battery to be easy at a C20 rate its capacity is 100AH, at a C5 rate that same battery might only have a capacity of 72AH and at a C40 rate it might be 120AH.

so you can change the C20 rate like you are trying to do, it is a overall measurment at a constant rate of discharge,

for charging a three stage charger doesnt realy apply to a 1C or 0.5C rate as it is an optimized method of charging that increases battery length and I would have to look it up but I think that rate is just for bulk , or its and average for the whole charge. three stage chargers are a relitively new thing when you think about it. so it may very well be that the #C rate only applies to constant output chargers. so when you notice the charge voltage and curent decreasing as it goes from bulk to adsorption to float that is just an optimization to gently bring the battery to 100% and reduce battery sulfication and such. this is why the adsorption phase is longer on a conventional battery and short (around 1 to 3 min) on a Li battery.

Steve