Generic LiFePo Battery Question......

[2kwik4u]

SO, there's a LOT of chatter here about converting from SLA-AGM batteries over to LiFePo4 batteries. Best I can tell they are a direct replacement, and work just the same as the SLA batteries they are replacing, with the added benefits of being lighter, and capable of handling more charges before becoming unusable. All great things.

I'm not working on a boat though, I'm rebuilding my youngests electric 4-wheeler. He has one of these: Dirt Quad - Razor This thing has (2) 12V 9Ahr wired in series to provide (I think) 24V and 18Ahr of capacity.

Existing packs look like this:


You can see one is failed and bulging. Probably why it quit taking a charge.


I put these standard SLA "Mighty Max" batteries in it two seasons ago, and they barely survived the first season, and he didn't ride it much last year. When I compare the SLA batteries to the LiFePO4 batteries the LI ones have a bit more runtime, but otherwise are the same voltage, shape/size/configuration.

These are the batteries we have now:

Amazon.com

These are the batteries I'm looking to get:

Amazon.com

I like the slightly longer run time of the new batteries, and honestly even at DOUBLE the cost, these are still under $100, so they seem like a good deal. As best I can tell they'll just bolt right in and work like normal. Would be nice if they would continue to hold a charge for longer than a season, and that would almost instantly generate the justification for the additional cost.

Am I missing anything? Will the Razor's existing battery charger handle these? If not, where do I find an appropriate 24V charger? Do I need to have any additional considerations for fusing or power wire sizing? Any random thoughts to share?

 

[adrianp89]

No idea to any of your questions. But what is max voltage the motor could handle? I'm sure you kid would love 48v lol.

EDIT: Series does not double amp hours, only voltage, and opposite for parallel. So series would be 24v and 9ah.

 

[2kwik4u]

No idea to any of your questions. But what is max voltage the motor could handle? I'm sure you kid would love 48v lol.

EDIT: Series does not double amp hours, only voltage, and opposite for parallel. So series would be 24v and 9ah.

Thanks for the correction. I was guessing at amp hours.

I'm not sure what the motor will handle. I would suspect I could get away with 36V, but it's plenty fast enough for him now.

Thought about getting him this, but at $750 it's a bit more cash than we want to spend currently. However if this machine holds up for a year, we might sell it to fund the new one. We did that with the oldests dirtbike. Bought it for $150, put about $200 in parts into it. Sold it for $300, and helped offset the $1,000 for the new one. I think I can get $350-$400 for the quad with good tires, paint, and batteries. Might be $250 into it all said and done.

Maybe I should find some more and make a side hustle out of this?!?!

 

[Bruce]

Being LIFEPO4 the batteries you are looking at likely have a nominal voltage of 3.2 volts per cell. I saw one review that states that a 16Ah version had 24 cells in it which would 4S6P six parallel banks of four cells in series. Typically LIFEPO4 has a maximum voltage of 3.6 volts per cell with 3.65 volts being dangerous. Each battery would have a full voltage of 14.4V combining for 28.8V in series.

This capacity chart could be helpful.



Your existing charger likely cuts out at 12.6 / 25.2 volts so it would not be much use. A 24V charger intended for Lithium Ion packs would charge to 29.4V which would be dangerous.

I am not seeing LIFEPO4 chargers as readily available items.

I use a charger like this to charge varios battery packs. Those batteries do not have balance ports but this charger would handle them well. You also need a power supply for the charger. I use an old 24 volt laptop charger as a power supply to it. You likely could use your old charger as the power supply. You would need to add an XT60 connector to the end of your power supply cable and to wires to your batteries. I believe all XT60 connectors are solder on but you can buy XT60 pigtails.

https://amzn.to/424fZ4u

 

[Bruce]

Adding two more points;

The battery is marketed as having a battery management system. That should protect from over charge so a 29.4 volt lithium ion charger should be fine. This one works well for my son’s 24 volt wheel chair. https://amzn.to/3F6EG6h

Those batteries are rated for a peak discharge of 20 amps. I was about to order a pair but 20 amps is not enough for my needs. It might be for yours. That is 480 watts max.

 

[Judge]

The one thing you need to look at is the charger output for the Razor. The LiFePO4's require 28.8V for a full charge (14.4V each battery). The stock charger may only put about 25. 2 - 27.2V if it was designed for AGM's. If that is the case, you will never get the LiFePO4's to full charge and you will leave some of the extra run time they would provide on the table.

Check the output on your charger. You may need to get a different one.

 

[2kwik4u]

@Judge @Bruce Thanks for the info guys.

That charger @Bruce listed has the same plug end on it as the Razor, so it looks promising. As best I can tell the Razor has a 350W motor on it, and I know is has a 30A fuse on the main power line between the two batteries. 24V*30A = 720W right? However that would be a failure mode!?!

I think these will work. Might give it a few more days to sink in before I pull the trigger.

 

[FSH 210 Sport]

In a series configuration, voltage doubles, amp hours stay the same, watt hours double. That’s because watts are equal to volts times amps.

SLA type batteries mostly talk about amp hours / Ah, that’s kind of voo doo math because the peak Ah rating is based on the 20 hour discharge rate, increase the discharge rate and the Ah’s go down. And the while the Ah’s are going down so is the voltage, so the watt hours are taking a bit of a dive.

Watts are true power, all that matters is watts or watt hours as that is the measure of work being done and or work storage. I X V=W

One of the big performance aspects of LiFePO4 batteries is their voltage curve stays much higher during high discharge rates, ergo higher watt hours.

SLA type batteries have a steady decline in voltage over a discharge cycle whereas LiFePO4 batteries have a flat voltage decline until the very end and the voltage drop is sharp.

So in theory the comparison looks like this.
SLA-AGM Batteries
12V X 9Ah=108 watt hours
24V X 9Ah=216 watt hours

LiFePO4
12.8V X 10Ah=128 watt hours
25.6V X 10Ah= 256 watt hours

But in fact the SLA batteries performance is lower than the math shows as explained above as the voltage is dropping so the watt hours are much less.

My assumption at this point is that you replaced the original set of batteries with the same type / chemistry that the bike came with? So the assumption is that the battery charger was designed for the SLA-AGM battery, and that’s not usually an issue for a LiFePO4 battery, except the logic in the charger thinks it’s an AGM battery and the absorption cycle is exponentially longer than is required for a LiFePO4 battery. An AGM specific charger is also good in that it will probably charge the AGM battery at .5C or 1/2 of the Ah rating of the battery, also not a problem for a LiFePO4 battery. But without knowing the charger specifics these are all assumptions.

Here is your problem with switching battery types, that higher deliverable wattage to the motor. This is the same issue with electric trolling motors, MinnKota recommends only 85% power usage (8.5 prop speed) with their trolling motors at a 100% duty cycle, otherwise the motor will burn up, meaning it will overheat and fail. MinnKota states that 100% power usage can be used for short periods, they won’t and probably can’t state a duration due to unknown delta’s between the motor temp and water temp / heat rejection rate, and even then at 100% power (10 prop speed) the motor will eventually fail due to overheat.

So, in the case of your kids little quad if 100% throttle is being used a lot there would be a very good chance the motor will fail. My recommendation would be to stay with the AGM type battery.

 

[2kwik4u]

@FSH 210 Sport

Appreciate the detailed response there.

Doing some digging around, it looks like the charging port goes THROUGH the controller, so I'm guessing the LiFePo batteries would never see the right voltage regardless of what charger I put on there. Here's the wiring diagram for the ATV.



I could certainly wire around that, but I would have to wire the charge port into the switch so that it's not live while he's riding, and only live while charging.

I think you're right. I might just go with the AGM batteries again for simplicity all around.

 

[Bruce]

@Judge @Bruce Thanks for the info guys.

That charger @Bruce listed has the same plug end on it as the Razor, so it looks promising. As best I can tell the Razor has a 350W motor on it, and I know is has a 30A fuse on the main power line between the two batteries. 24V*30A = 720W right? However that would be a failure mode!?!

I think these will work. Might give it a few more days to sink in before I pull the trigger.

The batteries have a BMS that should protect them from drawing more than 20 amps which could range from 576 watts down to 516 watts at 20% charge. The continuous discharge rating is 10 amps while the peak discharge rating is 20 amps.

It is possible that the battery is built from cells that have high storage capacity but low discharge rates or that the cells have a much higher discharge rate but the BMS is limited to 20 amps. Often the capacity of Chinese battery packs and cells is overrated.

It is likely that the 350 watt motor is overrated as well or at least peak rated.

There are review(s) that describe uses similar to what you plan so they may work well. If not they may be returnable.

The price is quite good. Typically I would pay more for the 48 cells than these battery packs cost.

 

[Bruce]

so I'm guessing the LiFePo batteries would never see the right voltage regardless of what charger I put on there.

Do you have a way to introduce a higher charging voltage and check the voltage going to the battery? I doubt the Razor controller has a charging controller built in. They likely include that in the charger itself.

 

[2kwik4u]

Do you have a way to introduce a higher charging voltage and check the voltage going to the battery? I doubt the Razor controller has a charging controller built in. They likely include that in the charger itself.

I don't currently.

I have some old PC power supplies in the basement that might get me the voltage, but they are likely not going to have any current rating.

Could I ohm in the input and output and get any idea of what circuitry is between to make an educated guess on if it would work?

 

[Bruce]

You might just check for continuity between the charger input and the battery terminals. If it is regulated I doubt that you would find continuity.

It is is regulated you could just wire the charging port directly to the battery.

 

[Bruce]

@2kwik4u I ordered a pair of those batteries. I will charge them with a standard 29.4 volt lithium ion charger, the one I linked above, and verify that the BMS prevents overcharging.

 

[2kwik4u]

@2kwik4u I ordered a pair of those batteries. I will charge them with a standard 29.4 volt lithium ion charger, the one I linked above, and verify that the BMS prevents overcharging.

Holy crap man. I hope you have an actual usage for them and arent just experimenting on my behalf.

 

[Bruce]

Holy crap man. I hope you have an actual usage for them and arent just experimenting on my behalf.

Range extension for my son's powerchair. They will be used well. Not enough amperage to power the chair alone but plenty to recharge the chair as he goes. I was planning to build a 24V 10Ah pack out of 21700s. The batteries you found are less work and cheaper.

 

[Bruce]

@2kwik4u, I was sorta rushed but printed a box for the pair of batteries and tried them out today. They do have overcharge protection so the cheap charger that I previously linked to did not harm the batteries. Interestingly they appeared to protect themselves around 28.3 volts and had a voltage of 27.5 when fully charged. The 28.8 volts that I expected seems to be a float voltage that drops to ~27.2 once the charge is removed.

 

[Judge]

@2kwik4u, I was sorta rushed but printed a box for the pair of batteries and tried them out today. They do have overcharge protection so the cheap charger that I previously linked to did not harm the batteries. Interestingly they appeared to protect themselves around 28.3 volts and had a voltage of 27.5 when fully charged. The 28.8 volts that I expected seems to be a float voltage that drops to ~27.2 once the charge is removed.

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They seem to be working properly. A float voltage is not applicable for 12V LiFePO4 batteries but once they are fully charged, the resting state can be ~13.6V.

There are some slight variations between LiFePO4 manufacturers but In order to get a full charge, 12V LiFePO4 batteries need to be charging at 13.9 - 14.6V to become fully charged. A good built-in BMS can also limit the charging voltage when it approaches the upper voltage limit for the cells.

A high quality charger (like the one I have in my motorhome) can be programmed to meet a specific batteries needs. Not all chargers can be programmed so that is one the BMS provides its own protection.

These are the charging parameters for the LIFePO4 batteries that I use in my motorhome.

 

[2kwik4u]

@2kwik4u, I was sorta rushed but printed a box for the pair of batteries and tried them out today. They do have overcharge protection so the cheap charger that I previously linked to did not harm the batteries. Interestingly they appeared to protect themselves around 28.3 volts and had a voltage of 27.5 when fully charged. The 28.8 volts that I expected seems to be a float voltage that drops to ~27.2 once the charge is removed.

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Good grief man.....if that's your "sorta rushed" product, I'm not sure we're worthy of a "I had some extra time" solution! That setup looks pretty awesome. I'm guessing it mounts under the chair, then when the chair is low, it provides a secondary battery you can swap over to? Like a reserve tank on the old motorcycles?

At any rate, I appreciate the testing and feedback. I'm headed to Amazon now to pick those up, as well as the charger. We'll figure out once it gets here how to wire the things into the quad and make sure the charger works. I think those are relatively easily surmounted obstacles at this point.

Thanks again!

 

[2kwik4u]

@Bruce Just sent the order for two of these batteries and the charger you linked. Should be here this week. I'll keep you posted on how they work out.

Thanks again for the help testing!