Drop in LiPo replacement for SLA

[wiredsim]

I've been a long time reader of this site and infrequent contributor, as I haven't had the time are money I would like to have had to put in to this hobby. However I recently acquired a used scooter from Craigslist and I need some new batteries for it. Here is my thread about it in the Scooter forum. I had some SLA's from a UPS, but they appear to be useless at this point. The scooter was designed to use 3x12volt 9ah SLA's and has a 36 volt controller with a 31.5 volt LVC (so the label says anyway).

Since decent replacement SLA's would run me at least $60 I've been looking at moving to a better battery chemistry. Long range isn't as important to me as performance, ease of charging and longevity. Plus I know that at 25 amps I wouldn't be getting 9ah out of those SLAs anyway. I'm leaning towards a LiPo pack, as I need to get a charger either way. My first though is to put in a 5S and a 6S pack of the same capacity for a combined 11s so that the LVC runs in a safe range for the pack. But I don't know if the fresh off the charger voltage would fry my controller. Would a 10s pack suffice with a 31.5 V LVC? Can I drain these to 80% DOD or lower and still get a decent cycle life? Do I have to use a BMS?

My other thought is to get some Bosch 36V Fatpacks, which at around $80 a 2AH pack is more expensive, however the charger is only $40, I don't have to worry about exploding packs, there is a basic BMS provided and they seem to be able to handle a high DOD and still give good cycle life.

So $200 for a Bosch 4AH pack plus charger.

Or

10s1p Lipo pack plus a charger like the GT A-6 for about $120 total.


LiPo comes out better with higher capacity and lower cost, but if I can't use as high of DOD then that extra capacity may not mean much to me. Also I'm not including any prices for BMS on the LiPo and the cycle life is going to probably be better on the Fatpacks.

Well I'm putting together these prices as I write this out and SLA is still coming out as the cheap way to go of course but the LiPo's aren't bad either. Is there any better budget lithium alternatives? I haven't found a source for used power tool packs in my area yet. I guess my big question is if I could just drop a 10S pack into my scooter and trust the existing controller to handle it? Anyone else make this upgrade?

Thanks!

 

[docnjoj]

I can't address the Lipo issue but have had extensive experience with Fatpacks. They are about 40.5 volts off the charger and are 10s2p. The cutoff would be around 31.5 volts through your LVC in the controller. Sounds good to me. Each Fatpack weighs about 2.5 lbs. They can pull 5-10 C without problem in fact the fuse within is 60 amps. Good luck with your decision.
otherDoc

 

[vanilla ice]

I think a 31.5v lvc is more appropriate for 9c lipo than 10 or 11. That is 3.5v per cell, and there isn't much capacity below that. I try to stop at 3.6 or 3.7 on mine.

If you want performance you could always buy a cheap 48v or 60v controller! Or just be mindful about state of charge and run 48v+ on the stock controller.

 

[wiredsim]

Thanks for the info guys!

Docnjoj, with your Fatpacks did you take them out of the case or find a way of connecting them without modification?

A lot of cell monitor I've seen will trigger at 3 volts per cell. Aren't Lipo considered a 3.7 volt nominal? What do you usually see off the charger and at 3.5 what DOD do you find that to be?

 

[docnjoj]

Hey Wiredsim. I actually did both. I used the original method of wiring in slide terminals from Home Depot at first and ran that for about 4 months. Definitely read the Fatpack thread. It is long but all good information. After I was sure I wouldn't have to bring them to Bosch for replacement I stated cuttin them up. I keep the cases and ends and use the electronics from one in the charger socket. I also made 18 volt (5s2P) but cutting the connector and paralleling them so I now run 4 FP in parallel and 2 FP's @ 18 volts in parallel. I then put them in series and both run and charge at 54 volts nominal. I have a good bit of info in the Fatpack thread but pay particular attention to the stuff Russell and John from CR have done as well. good luck and with FP you don't need any BMS! Very close to foolproof.
otherDoc

 

[wiredsim]

So I am planning on going with two Zippy or Turnigy 5000MAH 5s packs for now (whichever is cheapest) and add two more when I am able. I will be using a LVC alarm, though I am looking for a way of integrating that with my controller to shut it down. 10s sounds about perfect if I charge to 4.1.

What is a good budget Lipo charger? I don't really have the $$ for an iCharger unfortunately, however two Turnigy A-6-10's look like a decent option. I see the IMAX B6 is cheaper, but the reviews aren't nearly as good. Any good recommendations?

 

[dogman dan]

I bought two b6's expecting to soon have one, but both are working fine. Slow, at 50 watts though.

After you get some experience with em, you will know when to stop with a simple voltmeter , or wattmeter, to tell you when to stop discharging the lipo. I found em to have a pretty steady march down to the point where you want to stop, and then the cliff where voltage plummets comes later. But the top 80% of the discharge I found pretty easy to monitor by simply watching the pack voltage. This is with nice balanced cells though, of course.

One of the cheap beepers attached to each pack will help too of course.

 

[wiredsim]

Where do you see that cut-off point of the 80% then? 3.2 volts?

Also is that a good DOD for maximum life of the cells?

 

[dogman dan]

Uhh, I'm not really sure. I think my warning buzzers start going off a lot sooner, like 3.5 volts. On my whole pack, I think it's right around 48v when I'm running 15 s lipo when I have done 100%. So I stop at around 50v. That's from a starting point of 63v. If you are running 10s lipo, then you'd start out at 42v and would consider a 100% discharge to be around 35 v. So stopping at about 36-37 v resting would be about right I think. And never let it get below 30 volts under load.

Its generally agreed here that 80% max dod is a good number to aim for with most lithium. It seems to keep em more balanced, which in turn may extend cycle life, and most definitely makes em easier to avoid having one cell get over discharged compared to the rest of the pack.

 

[wiredsim]

Another thing I've been wondering about is how useful is a LVC in a constant usage situation. Because I am powering a scooter, anytime I want to move I am going to be on the throttle, so I am rarely ever going to see a resting voltage. My understanding is that you can sag a pack below the LVC voltage while you're drawing current, however its the resting voltage that gives you the best DOD info. So how would I know when to cutoff, or to make sure I am not cutting myself off too soon because its only sagging that low because of the current draw?

I suspect in my current situation with a fairly low amp controller I won't have enough of a voltage sag to run into this issue right now. But for a higher current situation, how do you deal with this?

 

[dogman dan]

You don't want to run any lipo below 2.7v ever, that includes sag under load. So you'd want to set an lvc that idealy monitored each cell, and cut off if any got too low.

With todays lipo c rates, the sag is pretty minimal so that's not such a big issue. But with a big enough controller, you'd see some. But not what you are used to with lower c rate batts.

Once you ride a bit while monitoring voltage, you get a pretty quick feel for how much sag you do get, and can do a mental adjustment. Or just correlate your dod with a voltage under typical load.