Quote:
Originally Posted by Natalya
Ok so let's say I plug my grid charger in... how long do I plug it in for to not hit 100% SoC? Maybe I just plug it in the night before and so it only goes for about 8 hours and doesn't max everything out?
Also, I'm pretty sure I have failed cells on at least 3 different sticks because I've got 3 taps that are always out of whack if the car sits more than 8 hours.
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One of the advantages of NiMH is that if you are going slow enough .. you can keep them on a 'trickle' charger for weeks or even months without issue.
That allows you to skip the more complicated path of actually getting a hard number type answer.
Soo .. depending on the rate of charge of your charger .. I'd recommend:
Option#1> For slow trickle charger putting out less than C/20.
In all but the hottest summer weather .. just plug it in and let it go .. not much need to worry about the top , it will just convert it to heat .. in all but very hot summer it will just get warm .. can be left on that for several days without an issue.
C/20 from a 100% new ~6.5Ah battery would be about ~325mA .. if you have say ~5Ah of capacity left in your older pack the same C/20 would be about ~250mA .. etc.
Option#2> Simple timer shut off based on charge rate.
Like the 8Hour you listed.
Faster than the trickle above, the timer shuts it off.
OEM the car uses about ~62% of the battery capacity .. it avoids bottom 20% and the top 18% .. from a new full 6.5Ah battery that is about ~4Ah .. if your older battery has about ~5ah left that's about ~3Ah.
The efficiency of NiMH charging varies with Rate, SoC, Temperature etc .. but in the bulk part of it .. before getting into the very top ~95% or so .. it's usually in the range of about ~90% .. the top starts to convert more and more to heat so the efficiency goes down considerably at the very top .. Sense you won't be doing any fancy cell level crap.. I'll assume a conservative ~80% overall battery efficiency as some of the cells reach the top and get a little warm.
That means
~4Ah stored @~80% = 5Ah need to be applied .. in ~8hrs that's a ~625mA rate.
~3Ah stored @ ~80% = ~3.7Ah need to be applied .. in ~8Hrs that's a ~470mA rate.
Beyond that amount it is just converting the energy to heat .. might be a little useful in winter to a have a warmer battery .. but not any other time of the year.
If your charger is a higher rate than the above ~625mA or ~470mA .. than a simple plug timer for something less than that ~8Hours is a easy control.
Quote:
Originally Posted by RedDevil
LiFePO4 batteries won't catch fire on a barbecue. And Atlanta should solve their main problem - getting damaged when you charge them below freezing.
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Just an FYI
LiFePO4 don't have to get damaged from charging bellow freezing temperatures .. I know that is a common simplification .. but the more accurate way of viewing it would be something like .. as it gets colder the rate of use (charge or discharge should be slowed) .. it isn't a line in the sand at freezing temps.
Not all LiFePO4 are identical .. there are some tweaks and adjustments that can be made in that flavor / family .. such adjustments can move the bellow numbers up or down a bit depending on details.
In a conventional charge / use method of CC-CV the continuous rate is used , not the ~10s pulse rate.
The attached is for 20Ah A123 Pouch style cells .. other flavors of LiFePO4 might be tweaked a bit above or bellow .. but the general pattern I'm describing is the general slowing as it gets colder .. Even warmer than freezing it should be slowed some .. but with more slowing charging can continue to be done bellow freezing without issue.
Of course normal cell terminal voltage limits still apply no matter what the recommended current flow rate is at some temperature .. and sense the internal resistance of the LiFePO4 cells increases significantly as they get colder, this voltage effect can sometimes dominate the charge / discharge usage limitations before actual current (AMP) flow limitations listed in the attached are reached.