# Basic LiFePO4 charging question



## RogerK (Dec 20, 2011)

I am doing Honda Goldwing conversion. I did a 66 MG as my last conversion with Led Acid. Used it a few years and decided to strip it and use many parts in the Honda. Since it is a motorcycle and I would like to get some range, I plan to use the new Lithium Iron Phosphate batteries. I'm planning on about 70V (60ah). So I've been studiying these. Apparently it is critical you don't overcharge or over discharge these cells or they get damaged. So I was reading about all the BMS units you can use to monitor and control charge and dischage. Then I ready about charging in a series pack, and how the chargers can "protect" the batteries. I do not see how this is possible if you had a few cells quite low, then you could easily over charge others since you would have not yet reached max voltage. 

I'd like to know the facts of LiFePo charging and battery protection. Can they be charged safely in a bank? Can a too low discharge can kill a cell? If so, is a BMS the only solution? Would a too low battery then stop operation of the vehicle? Can you get a warning first. I'd like to get some of these facts prior to investing lots of money into batteries I could ruin. This happened to a lesser degree to my led acid batteries in my MG.

The BMS that looks the best so far is: http://www.cleanpowerauto.com/MiniBMS.html
Thoughts?
Thanks,
Roger


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## DaveAK (Jun 28, 2009)

I run 24 60Ah CALB cells in my bike conversion with the MiniBMS. The BMS also helps on charging as well as discharging to prevent what you write about. When charging the BMS will shunt current around the higher cells to allow the lower ones to catch up. When any one cell reaches full voltage the MiniBMS can cut off the charger.

When discharging if any cell drops below minimum the MiniBMS will send an alarm signal which can be used to sound a buzzer, light an LED or cut back the throttle, (or any combination of those).


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## RogerK (Dec 20, 2011)

Thanks for the reply Dave. I thought the BMS I posted seemed like a good candidate. But I would like to hear from someone who charges these batteries as a bank without monitoring. Many people do it this way. Not sure if they are paying for it, and thats what I would like to know. Also, I wonder if the car companies have any BMS on thier battery packs (like the Leif or Roadster). Any comments on this?
Roger


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## dladd (Jun 1, 2011)

RogerK said:


> Thanks for the reply Dave. I thought the BMS I posted seemed like a good candidate. But I would lie to hear from someone ho charges these batteries as a bank without monitoring. Many people do it this way. Not sure if they are paying for it, and thats what I would like to know. Also, I wonder if the car companies have any BMS on thier battery packs (like the Leif or Roadster). Any comments on this?
> Roger


a quick search here will find many folks who charge in series without any monitoring. A few use a 1/2 pack monitor. It's been pretty well shown that a set of Lithium cells that are balanced stay balanced. Stop the charge a bit short, discharge to less than 80%, they seem OK.

All OEM's use a BMS.


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## dougingraham (Jul 26, 2011)

RogerK said:


> Thanks for the reply Dave. I thought the BMS I posted seemed like a good candidate. But I would like to hear from someone who charges these batteries as a bank without monitoring. Many people do it this way. Not sure if they are paying for it, and thats what I would like to know. Also, I wonder if the car companies have any BMS on thier battery packs (like the Leif or Roadster). Any comments on this?
> Roger


So far all the OEM's have some sort of BMS but then they aren't using LiFePo4 cells. The cells the OEM's are using need rather a lot of monitoring to prevent problems. The Leaf battery actually has a lower wh/kg rating than the LiFePo4 battery pack I am putting in my car because of all the battery management stuff. My battery pack is 99.88 wh/kg counting everything and the Leaf pack appears to be 80.81 wh/kg. The cells themselves are about 155 wh/kg for the Leaf and my GBS batteries are about 105 wh/kg. The difference is all in the packaging.

If you get your LiFePo4 cells in balance they will generally stay in balance. If you don't abuse them it looks like they will last a long time. Don't overcharge which is pretty easy and don't over discharge which can be accomplished with a State Of Charge monitor that simply counts amp hours in and out and you should have good success.


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## RogerK (Dec 20, 2011)

Thanks for the replys. My experience with my MG and the 8 led acid did show lots of imbalance issues. Partly because I has 2 sizes of batteries, but mainlyy because I sucked the bank down to nothing several times trying to get home. 

Good to know the LiFePo batteries are more robust that I had thought. Or at least robust if you manage them. My latest thought is to connect wires to all the batteries then to a plug. Once a month I would plug in a monitor unit and test the pack. If I want to get elaborate, I could build an off line balancer with a plc and relays like I did with the MG. Actually I still have that equipment, so why not. 

Since I live in Minnesota, the goldwing will be seasonal. I may fine I balance in the Spring, then the late summer and I am good. This winter I could even venture to drive in December. With the full ferring, and no snow yet, I could get away with it.


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## RogerK (Dec 20, 2011)

I thought I'd share a picture of the MG I did. Sort of a shame I tore it down for parts. Oh well, off to the next project.


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## EVfun (Mar 14, 2010)

The trick about LiFePO4 cells, that allows fairly long series strings to be charged without a BMS, is that don't self discharge. (unless they have already been damaged by over-discharge or over-charge) A second reason a Li pack can work without a BMS is that no harm comes to the cells by not fully charging regularly. (not fully charging seems to even increase their life) 

If you balance a pack at the top they will all reach the top at very close to the same time for quite a long time. All you need to do is check, once-and-a-while, to make sure none are creeping close to 4 volts near the end of charge. Oh, you also need some way to make sure you *never* over-discharge the smallest cell (they may vary by 2% in capacity) with an amp hour counter or other device.

If you choose to bottom balance the pack you need an adjustable charger so you can terminate charge at a point where the smallest cell isn't going over 4 volts. You need to check regularly to make sure any slight drift doesn't allow the highest voltage cells to creep up past that point. One the discharge side you can drive to the point of crawling with a bottom balance pack as they are going low voltage together.

You cannot use voltage as an accurate method of determining a cells state of charge except at near 0 charge and near 100% charged. The voltage drops off fast right near dead and rises rapidly very near full. In the middle the voltage is almost flat.

I don't have a horse in the BMS debate. I think they have value, but introduce their own new failure points too. I have run my cells with a BMS and now without. If my car had a higher voltage pack and saw use during more extreme weather conditions I may not have removed the BMS. With my small pack and mild climate I never saw the BMS do any good in the first year, so I ditched it.


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## StanSimmons (Sep 3, 2011)

One thing that I've seen recommended for LiFePO4 is to store the cells at about a 50% charge. That is the state of charge that most cells seem to be coming from the factory with, and several people have stored the cells for as long as two years at that SOC with no damage to the cells. Keep that in mind when you store your bike for the winter.

I won't be putting a BMS on my bike, as I plan on staying well away from the "knees" at either end of the charge curve. I see all the extra wiring as more points of failure in the system. I will be putting in a simple 1/2 pack monitor with small fuses at the battery connection points. 

Here in DFW, TX, I'm more likely to have a longer time off the bike in the heat of August than in the cold of January. Your experience in Minnesota will be much different I'm sure. One thing about LiFePO4 cells is that they really don't like to be charged below freezing. If you do any cold weather riding, you will need either insulated/heated battery boxes or a heated garage. 

I'll be watching your Goldwing build with interest... I am at the early stage of converting a Honda ST1100 to electric and I haven't finalized how I'll be attaching to the shaft. I'll be pulling the gas engine out after Christmas and making my battery/controller/motor choice in early Jan. 2012.


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## RogerK (Dec 20, 2011)

Let's keep in touch as we both do these Honda conversions. I like the 1/2 pack monitor and not having the complexity of the BMS. Then just have a smart LiFePo charge that will charge the pack safely. I think I would still add the wires to a connector to I can check the pack once in a while (and maybe balance from there is need be). Otherwise it means tearing the bike down somewhat to access all the batteries. 

I would be currious what you are planning for a system voltage and AH rating? I am currently looking at a 70V system using 60AH batteries. From what I've calculated, I shoud easily get 30 miles, and I think I am good with that for now. The long range goal of pulling a generator in cases of a long ride. 

The motor I have is from my MG conversion. It is an 8" warp by Advanced DC motors. The rating is 17.5 HP. So plenty for the Goldwing. I am just now looking at mounting and coupling. The motor will sit in the belly of the frame, then the shaft comming out the front, near the fork, with a 3:1 timing belt coupling to a shaft that is in line with the driveshaft to the wheel. 

Here are a few pictures:


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## StanSimmons (Sep 3, 2011)

RogerK said:


> Let's keep in touch as we both do these Honda conversions. I like the 1/2 pack monitor and not having the complexity of the BMS. Then just have a smart LiFePo charge that will charge the pack safely. I think I would still add the wires to a connector to I can check the pack once in a while (and maybe balance from there is need be). Otherwise it means tearing the bike down somewhat to access all the batteries.


If you add wires to monitor each cell, be sure to fuse them near the cell. If a wire gets rubbed thru and isn't fused you can damage that cell pretty quickly.



RogerK said:


> I would be currious what you are planning for a system voltage and AH rating? I am currently looking at a 70V system using 60AH batteries.


I haven't made my decision yet on cells/controller/motor. After I pull the ICE I will be mocking up the motor and cells to see what configuration will work best. I am looking at the new smaller Sinopoly LFP60AHA(B) cells, in a 32 cell/102v/6kWh pack they would save almost 9 liters of space and 9.5 kilos over the older 60Ah models. A 32 cell pack should get me somewhere between 35 and 40 miles at 80% discharge. I hope to be able to fit more or larger cells for more range.



RogerK said:


> From what I've calculated, I should easily get 30 miles, and I think I am good with that for now. The long range goal of pulling a generator in cases of a long ride.


 I too am looking at a trailer for long distance, but I'm leaning more towards a pusher than a generator. A two point tube and socket mount would effectively turn the trailer and bike into a trike but I would lose some maneuverability. My current thoughts are to get a used small diesel car from Japan and cut the rear off of it, turning it into a trailer (for DOT purposes). Anything like that is at least a year away for me.



RogerK said:


> The motor I have is from my MG conversion. It is an 8" warp by Advanced DC motors. The rating is 17.5 HP. So plenty for the Goldwing. I am just now looking at mounting and coupling. The motor will sit in the belly of the frame, then the shaft coming out the front, near the fork, with a 3:1 timing belt coupling to a shaft that is in line with the driveshaft to the wheel.


Interesting, I never would have thought about having the pulley system at the front of the bike. My current plan is to have the motor vertical, with a 90 degree gear box fabricated to attach to the shaft. If that won't fit, then have the motor in the belly longitudinally with the motor shaft to the rear and use a toothed belt system to the driveshaft. I plan on using the existing radiator for controller and charger cooling.

My build planning thread is: http://www.diyelectriccar.com/forum...ting-large-motorcycle-honda-st1100-66217.html


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