# Charging while using LiFePO4



## dimitri (May 16, 2008)

whelmed said:


> Preface - building a car which will have a micro-generator in the boot (or front, not sure yet).
> 
> I'm not sure if it makes more sense to 'charge' LiFePO4 batteries at the same time they are being used or it I should just put the charger in parallel so that the load is reduced. Would there be load balancing issues if I did the latter? Any thoughts appreciated.


Isn't it the same thing anyway? There is only one main circuit in EV, so adding genset or charger would always make them parallel to the battery, what am I missing here?

At any given moment if genset makes more current than motor is using, then remaining extra will go into the battery, if less, then less current will be coming out of the battery, offset by genset current.

LiFePo4 can be safely charged up to 3C, so I doubt you will ever hurt the battery with onboard genset as long as you don't overcharge them. You just need to stop genset in time. BMS can be designed to signal genset relay the moment first cell reaches HVC level, to immediately stop genset.

Hope this helps.


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## whelmed (Jun 29, 2009)

Yeah, that does help. Not sure what you mean by 3C. 

Correct me if I'm wrong here, but charging means that the charger is outputting a higher potential energy (V) than is currently in the batteries. Now, if the EV was running, by putting a higher voltage, wouldn't that create a much higher drag for the charger? At peak I'd be able to put out 7kW from the chargers which has me worried that I'll just blow the circuit if I try to accelerate hard when it's on.


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## dimitri (May 16, 2008)

whelmed said:


> Yeah, that does help. Not sure what you mean by 3C.
> 
> Correct me if I'm wrong here, but charging means that the charger is outputting a higher potential energy (V) than is currently in the batteries. Now, if the EV was running, by putting a higher voltage, wouldn't that create a much higher drag for the charger? At peak I'd be able to put out 7kW from the chargers which has me worried that I'll just blow the circuit if I try to accelerate hard when it's on.


3C means 3 times of rated Capacity, i.e. if your cells are 100Ah, then you can charge them with current up to 300 Amps.

Whatever kW your genset puts out will be absorbed my motor/battery, so it will probably just run at max capacity, which is what you want, isn't it?

If its rated to produce 7kW then it shouldn't blow a circuit doing it.

Voltage will drop as current rises, so whole thing will balance out as long as nominal voltages were matched up to begin with.


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## dimitri (May 16, 2008)

After I read it again, I realized that you mean to plug charger's AC input into genset AC output, right? In this case, charger will not pass more power to the battery than its rated for, just like if you had it plugged into mains line. However, if genset generates less than charger is capable of, then charger will probably trip and stop working, depends on how smart the charger is.

Downside of this approach is that you convert DC generated by genset to AC via inverter and then back to DC via charger. If you can find or make a genset to produce correct DC voltage for your pack, you can connect genset directly to the pack via diode, so current only flows from genset to the pack and not back to genset.


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## whelmed (Jun 29, 2009)

dimitri said:


> If you can find or make a genset to produce correct DC voltage for your pack, you can connect genset directly to the pack via diode, so current only flows from genset to the pack and not back to genset.


That's more along the lines of what I've been thinking lately. Try to get a high-voltage alternator / generator and just hook it up via a belt to a small diesel engine and run it through a diode bridge. Would have a few monster capacitors on there as well. I was kinda hoping the BM for LiFePO4's would be able to regulate the charge a bit, but I've also started thinking that I may start this project just using a few Pb-acid batteries as a proof of concept.


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## dimitri (May 16, 2008)

What's there to regulate? Just charge at max current until specific SOC, say 85%-90% of what you normally charge with home charger, then stop the genset. No need to balance with genset, that can be done at home with "smarter" charger. IMHO, onboard genset should always be run at max capacity and then shut off when enough energy was generated to get you going.

IMHO, using genset with Lead Acid is just a bad deal all around, everything is heavy, huge Puekert losses, still having stinker on board, might as well just drive a stinker. I think series hybrid only makes sense with good batteries.


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## Lordwacky (Jan 28, 2009)

dimitri said:


> What's there to regulate? Just charge at max current until specific SOC, say 85%-90% of what you normally charge with home charger, then stop the genset. No need to balance with genset, that can be done at home with "smarter" charger. IMHO, onboard genset should always be run at max capacity and then shut off when enough energy was generated to get you going.
> 
> IMHO, using genset with Lead Acid is just a bad deal all around, everything is heavy, huge Puekert losses, still having stinker on board, might as well just drive a stinker. I think series hybrid only makes sense with good batteries.


I'm liking this. I have found very little information on gesets in general. Once my build is done I think I would like to build a Genset trailer that I could pull for longer trips. My build will be using LiFePo batteries and I was wondering if doing that would hurt the more delicate batteries, cause imbalance issues or what not. I am likely going to use a 30 amp manzanita charger and I was thing I could just put a 6kWatt ,220 volt generator on a trailer and plug in the charger while I was going, of course 30 amps will not even nearly cover the current draw needed so I'm not sure how much range I'd gain. Im just wondering if this is even feasible.


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## dimitri (May 16, 2008)

Lordwacky said:


> My build will be using LiFePo batteries and I was wondering if doing that would hurt the more delicate batteries.


I think its a misconception to think of LiFePo4 as "delicate" just because they are expensive 

As long as you are within correct voltage limits they can take pretty good abuse levels, probably more than floodies can. Especially during charging, I don't think any Lead Acid battery can take even 1C charge current, but LiFePo4 can take up to 3C, so its unlikely you will hurt them with the genset as long as you watch the voltage and shutoff in time.


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## COS (Dec 23, 2008)

Lordwacky said:


> I'm liking this. I have found very little information on gesets in general. Once my build is done I think I would like to build a Genset trailer that I could pull for longer trips. My build will be using LiFePo batteries and I was wondering if doing that would hurt the more delicate batteries, cause imbalance issues or what not. I am likely going to use a 30 amp manzanita charger and I was thing I could just put a 6kWatt ,220 volt generator on a trailer and plug in the charger while I was going, of course 30 amps will not even nearly cover the current draw needed so I'm not sure how much range I'd gain. Im just wondering if this is even feasible.


 
It's completely feasble. LiFePo4 packs don't get unbalanced that easy. As long as at some point during a few days you plug in your batt pack to charge to "Full", your BMS will correct/balance your cells.
________
AntiCrisis_Girl


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## Bowser330 (Jun 15, 2008)

Lordwacky said:


> I'm liking this. I have found very little information on gesets in general. Once my build is done I think I would like to build a Genset trailer that I could pull for longer trips. My build will be using LiFePo batteries and I was wondering if doing that would hurt the more delicate batteries, cause imbalance issues or what not. I am likely going to use a 30 amp manzanita charger and I was thing I could just put a 6kWatt ,220 volt generator on a trailer and plug in the charger while I was going, of course 30 amps will not even nearly cover the current draw needed so I'm not sure how much range I'd gain. Im just wondering if this is even feasible.


The way I have been told to look at calculating rough range estimates is examining the kilowatts...

Your average weight/drag car should take about 20kw to keep a 70mph speed.

Lets say you have a 20kwh battery pack, this would mean, simply, you would have 70 miles of range (100%DOD) (at 70mph constant)

If you were to run the 6kwh genset at the start, you would be adding 30% extra power to the pack, so your range would increase by 30%, which makes your 100%DOD range 91 miles. Not bad...

the best will be when the A123 packs are cheaper...you could drive around for over 100-200 miles per charge, and just stop at a gas station (on a roadtrip), charge up your pack, and fill up your emergency genset gas tank, and be on your way...10-15 minutes max, a little longer than a regular gas fill up..


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