Lithium (LFP) version Lead-Acid batteries
Discussion
First of all, apologies if you've read this before, as I've posted it on 10/10s as well. Also, sorry that this is quite long with indistinct questions.
I am considering the virtues and drawbacks of using lithium batteries in our single seater F3 cars, and need some advice of a slightly more technical nature than most dealers can provide. I do prefer to know how and why something works (or doesn't) before committing (or rejecting) it. The fact that other people use them and get on with them doesn't mean very much to me, as everyone has their own expectations, working styles, needs etc.
Current car comes with a Deka 26Ah (10 hour rate), 365CCA (presumably at 0°C?), and weighs 9.8kg. We know it would do the job, and we are confident we could just about recharge it fully (or at least enough) between sessions even in the worst case scenario of the race being only a couple of hours after qualifying. Sessions are mostly around 25 minutes, but I suppose a test session or a delayed start etc mean that at least 40 minutes per charge is required. There is no alternator. We can't fit one (not enough room, and not allowed in the regulations).
We do not know the current draw of the vehicle, but I would estimate it to be around 12A plus whatever pulse currents the injectors and coils need that my old ammeter doesn't detect. I did measure a very similar cars draw last winter, so I don't think it'll be a million miles apart.
So, how to 'work out' (guess) what the equivalent is in Lithium. Given that the acceptable C rate of Lithium is much higher, then the rated Ah is much more likely to be true at ~12A draw.
Using Peukert's Law (bit out of my depth here), assuming a 26Ah 10hr rating, a 14A current draw and a Peukert constant of 1.2 (for a new AGM battery in reasonable temperatures), then the 'REAL' capacity is about 18.6Ah.
Now, I believe that the constant for LiFePo4 is closer to 1.01, so to get the same 'life' from one under the same conditions, I'd need an 18.6Ah lithium battery. Does that stack up? Could I even get away with less than that, because the voltage will stay higher for longer on Li than with PbA?
Anyway, that would probably mean something like a Super B 15P battery (15Ah, £700, 2.6kg), or a Racing Batteries RCB750 (13Ah, also £700 + charger, 2.8kg). Will I finish my races with those? The cranking amperage is sufficient on both of those (double that of the AGM battery).
Carrot Cycles sell a Shorai LiFePo4 battery with an EQUIVALENT Ah of 27, that weighs just 1.9kg, has a CCA of 405A (still more than the Deka), and is only £240. What's the catch? They aren't certified for motorsport officially, but that probably just means they haven't double the price and advertised it to racers yet. Or does it?
Further to the above (and ultimately this is a question that I would like clarifying/confirming, rather than a statement), regarding starting the car, it is not the CCA that starts the car as such, but the amp draw of the starter motor and the voltage of the battery at that current draw. Since lithium copes better with high rates of discharge, they maintain a higher working voltage at the same current draw. So, if a starter draws 100A, and a lead acid 400CCA battery drops to 10V whilst cranking, then that is 1kW. If a similar lithium battery does the same thing, it might only drop to 12V, so it can provide 1.2kW of starting power - i.e. it starts the car quicker/easier etc. As such one could take published CCA figures for a lithium battery and multiply it by a factor (lets say 1.2) to compare to lead-acid.
Therefore the 365A Deka could be considered the same as a 300A lithium, all other things being vaguely equal?
Next - charging. Super B only sell a 2.5A charger, so I doubt I could recharge a full discharged battery in 2 hours (worse case). They recommend a 14A charger, but don't sell one!!!! Are the technologies of the different brands so varied that I must not use a charger from a different brand? Racing Batteries are the only lithium people I've read about that are either honest or stupid enough (no idea which) to recommend a normal lead-acid charger... I know our 'clever' CTek charger will kill lithium batteries.
I will charge the batteries by plugging the charger into the same Anderson plug that the slave battery uses (the slave battery would be PbA - is that an issue at all?), so I do not want a battery that requires a funny little plug that fits in to the side of the battery casing. It must charge via the normal terminals, and hence cell balancing stuff must work with those.
As you might be able to tell, I know a little bit about batteries and electrics, but only enough to get me in a muddle and confuse myself. A little knowledge is a dangerous thing. I'd love to hear the thoughts, counter-arguments and perhaps calculations of someone who knows more than me.
If I had an alternator, I'd just buy a cheap battery with just enough CCA to start the car three times in a row. Bugger.
The batteries go under the seat, on the floor of the car, millimetres from the track, so very harsh environment in terms of shock, heat and vibration. I can and would add foam padding as much as possible.
Sorry for the long post and many questions. I'm not very good at explaining things concisely.
I am considering the virtues and drawbacks of using lithium batteries in our single seater F3 cars, and need some advice of a slightly more technical nature than most dealers can provide. I do prefer to know how and why something works (or doesn't) before committing (or rejecting) it. The fact that other people use them and get on with them doesn't mean very much to me, as everyone has their own expectations, working styles, needs etc.
Current car comes with a Deka 26Ah (10 hour rate), 365CCA (presumably at 0°C?), and weighs 9.8kg. We know it would do the job, and we are confident we could just about recharge it fully (or at least enough) between sessions even in the worst case scenario of the race being only a couple of hours after qualifying. Sessions are mostly around 25 minutes, but I suppose a test session or a delayed start etc mean that at least 40 minutes per charge is required. There is no alternator. We can't fit one (not enough room, and not allowed in the regulations).
We do not know the current draw of the vehicle, but I would estimate it to be around 12A plus whatever pulse currents the injectors and coils need that my old ammeter doesn't detect. I did measure a very similar cars draw last winter, so I don't think it'll be a million miles apart.
So, how to 'work out' (guess) what the equivalent is in Lithium. Given that the acceptable C rate of Lithium is much higher, then the rated Ah is much more likely to be true at ~12A draw.
Using Peukert's Law (bit out of my depth here), assuming a 26Ah 10hr rating, a 14A current draw and a Peukert constant of 1.2 (for a new AGM battery in reasonable temperatures), then the 'REAL' capacity is about 18.6Ah.
Now, I believe that the constant for LiFePo4 is closer to 1.01, so to get the same 'life' from one under the same conditions, I'd need an 18.6Ah lithium battery. Does that stack up? Could I even get away with less than that, because the voltage will stay higher for longer on Li than with PbA?
Anyway, that would probably mean something like a Super B 15P battery (15Ah, £700, 2.6kg), or a Racing Batteries RCB750 (13Ah, also £700 + charger, 2.8kg). Will I finish my races with those? The cranking amperage is sufficient on both of those (double that of the AGM battery).
Carrot Cycles sell a Shorai LiFePo4 battery with an EQUIVALENT Ah of 27, that weighs just 1.9kg, has a CCA of 405A (still more than the Deka), and is only £240. What's the catch? They aren't certified for motorsport officially, but that probably just means they haven't double the price and advertised it to racers yet. Or does it?
Further to the above (and ultimately this is a question that I would like clarifying/confirming, rather than a statement), regarding starting the car, it is not the CCA that starts the car as such, but the amp draw of the starter motor and the voltage of the battery at that current draw. Since lithium copes better with high rates of discharge, they maintain a higher working voltage at the same current draw. So, if a starter draws 100A, and a lead acid 400CCA battery drops to 10V whilst cranking, then that is 1kW. If a similar lithium battery does the same thing, it might only drop to 12V, so it can provide 1.2kW of starting power - i.e. it starts the car quicker/easier etc. As such one could take published CCA figures for a lithium battery and multiply it by a factor (lets say 1.2) to compare to lead-acid.
Therefore the 365A Deka could be considered the same as a 300A lithium, all other things being vaguely equal?
Next - charging. Super B only sell a 2.5A charger, so I doubt I could recharge a full discharged battery in 2 hours (worse case). They recommend a 14A charger, but don't sell one!!!! Are the technologies of the different brands so varied that I must not use a charger from a different brand? Racing Batteries are the only lithium people I've read about that are either honest or stupid enough (no idea which) to recommend a normal lead-acid charger... I know our 'clever' CTek charger will kill lithium batteries.
I will charge the batteries by plugging the charger into the same Anderson plug that the slave battery uses (the slave battery would be PbA - is that an issue at all?), so I do not want a battery that requires a funny little plug that fits in to the side of the battery casing. It must charge via the normal terminals, and hence cell balancing stuff must work with those.
As you might be able to tell, I know a little bit about batteries and electrics, but only enough to get me in a muddle and confuse myself. A little knowledge is a dangerous thing. I'd love to hear the thoughts, counter-arguments and perhaps calculations of someone who knows more than me.
If I had an alternator, I'd just buy a cheap battery with just enough CCA to start the car three times in a row. Bugger.
The batteries go under the seat, on the floor of the car, millimetres from the track, so very harsh environment in terms of shock, heat and vibration. I can and would add foam padding as much as possible.
Sorry for the long post and many questions. I'm not very good at explaining things concisely.
No 1: stop guessing and measure the actual current draw of the electrical system at various engine speeds (can do this in neutral whilst not moving as current will really only increase with rpm and probably not that much) - buy/borrow/steal a current clamp and measure it. That way you can actually base your maths on some proper data. When you consider the cost of LiPo's, the extra money you will need to buy a current clamp is minimal
No2: The critical issue for starting is the battery series resistance. As you draw a current the voltage falls due to this resistance. If you measure your starting current (will require a large current clamp, but you can buy them fairly cheaply, such as http://uk.rs-online.com/web/p/current-transducers/... ) Once you have determined the operating parameters of your exisiting system then specifying an alternative becomes much easier.
No3: Your LiPo cell suppliers should be able to tell you
Series R
Max discharge - continuous
Max discharge - peak intermitent
Optimum Charge profile (CC, CV etc)
They should also be able to furnish you will a suitable charging system as well.
No2: The critical issue for starting is the battery series resistance. As you draw a current the voltage falls due to this resistance. If you measure your starting current (will require a large current clamp, but you can buy them fairly cheaply, such as http://uk.rs-online.com/web/p/current-transducers/... ) Once you have determined the operating parameters of your exisiting system then specifying an alternative becomes much easier.
No3: Your LiPo cell suppliers should be able to tell you
Series R
Max discharge - continuous
Max discharge - peak intermitent
Optimum Charge profile (CC, CV etc)
They should also be able to furnish you will a suitable charging system as well.
Am just after the principles. The issue of whether my guesstimates (most of which are based on measurement).
LFP people for racing tend not to know how to compare with Lead-Acid properly. I've had one think that the Ah rating of a lead battery holds true even in the range of 2C discharge etc.
So, can I compare battery technologies with any confidence? Can I do that with the basic equations I've used. Will mixing LFP onboard with PbA starter battery cause ANY problems?
Etc etc.
LFP people for racing tend not to know how to compare with Lead-Acid properly. I've had one think that the Ah rating of a lead battery holds true even in the range of 2C discharge etc.
So, can I compare battery technologies with any confidence? Can I do that with the basic equations I've used. Will mixing LFP onboard with PbA starter battery cause ANY problems?
Etc etc.
Hi I have fitted LiPo batteries to my 750Formula car,as I only do short clubmans races I use two 4 cell packs 5000 mah 30C in parallel,this gives me 300 amps cranking and I am using a standard Fiat starter motor. The pair weigh 1.1 kg and I have done twenty engine starts with no problems.I use a Bantan BC6 to balance/charge the cells, also paid a lot less for them.
Tony
Tony
tonybraz said:
Hi I have fitted LiPo batteries to my 750Formula car,as I only do short clubmans races I use two 4 cell packs 5000 mah 30C in parallel,this gives me 300 amps cranking and I am using a standard Fiat starter motor. The pair weigh 1.1 kg and I have done twenty engine starts with no problems.I use a Bantan BC6 to balance/charge the cells, also paid a lot less for them.
Tony
So you're effectively running 10Ah? How long are your races, and what is the car running (pumps, loggers, injection etc)? I'll have a look at the Bantan charger. Tony
Cheers.
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