Help testing 24V 20AH LiFePO4 pack

[akumar55]

Hello all, I'm new to the forum so I hope I'm posting this in the right place. Any help is greatly appreciated.

A few months ago I purchased a battery pack from ebay that came from Sharp JH-AB03 backup power system (Link). I'm trying to figure out how to use this thing in my 24V lawn mower and maybe an ebike later. Based on what I can tell so far, this is some type of 8s configuration with a built in BMS. I charged it using a CC/CV power supply to 28.8V and then discharged it while taking readings and everything looked good.

The questions I have are about how to use it on a regular basis and the capabilities of the built in circuitry.

1. Series packs usually require balancing. Is it OK for me to assume that this function is provided by the BMS? Any way to test it? Would I be OK just charging it with a CC/CV type supply set to 28.8V and just leaving it plugged in?

2. I'm not sure if this BMS has a low voltage cut-off built in or if I need to add one. Suggestions on a safe way to test this? I ran the test to 22V but I'm sure the 2.75V per cell would be too high. I can probably open up the pack and test individual cells while discharging to see when/if it cuts power.

3. The label on the battery pack says "Output: DC25.6V 20A". I'm guessing that the 20A is the BMS limit since these batteries are typically capable of much more. I'm not sure if the BMS would limit the current to 20A or just get damaged if more than 20A was drawn. Again, any suggestions on how to test it? Would it be OK to hook this up to a motor directly with a switch or do I need to add in some current limiting capability.

 

[999zip999]

What mower do you have ? What is it amp draw. Can it give the ampage the mower needs. Most bms have hvc per cell, lvc per cell and balance mode. It will cutoff if over loaded as mowers can demand. But E.S. has mower experts.

The mow I do the mow I like.

 

[amberwolf]

It probably shuts the pack off if more than 20A is drawn from it.

You would have to test for LVC and balancing, but most BMS's entire purpose is to do those things, so if it doesnt' have that I'd be surprised.

If you look up the part numbers printed on the various multi-legged chips on the board, you may find some details about the BMS thru the datasheets for those chips.

LiFePO4 BMS may have cell-level LVC at 2.0-2.8v, depending on how conservative it is designed to be. 3.2v is "nominal", and where most of the capacity is, so some really conservative ones might cutoff at 2.8-3v.

Balancing probably happens during charging at about 3.6-3.65v, so until cells get that high you probably wouldn't see it happening.

Your mower may well draw more than that at startup even without load of lawn under it, and may draw lots more than that momentarily when you hit tough spots. You'll need to test the mower with it's existing setup to find out first.

If you want to add current-limiting ability, then if hte mower does nto already have a controller built in, you can add one; it's probably a simple PM DC brushed motor and can use a simple brushed motor controller. I would oversize the controller to perhaps twice what the mower is rated "continously" for so you can be sure it won't be damaged by heavier-than expected loads, and still be able to do the mowing you need.

There are a few threads about other people's mower and trimmer conversions and builds, if you poke around the forums, if you are interested in what other stuff people have already done and what problems they encountered that you may not have thought of yet.

 

[akumar55]

Guess I should've included the mower details. Its an old black and decker CMM1200 Link. It uses 2 small 12v batteries and is apparently 1200 watts. I haven't figured out a way to test the current draw of the motor in various conditions; eg starting, no load, transient loads.

I'm hoping that the 1200 watts is a peak value and regular draw is much less. I am planning to add a small capacitor bank to aid the starting current draw and transient conditions when it hits a rough patch etc.

 

[amberwolf]

(I added more stuff to my post while you were posting, so you might go back and read that too).

To test the mower's usage, you can use a wattmeter if you have one, or an ammeter (clamp-on or wire-in-series), or a multimeter with ammeter function. Check the manual for the meter to see exactly how to set it up to measure DC current, and to be sure it will actually be able to measure as high as the mower is expected to go.


Note that 1200w / 24v is 50A, so mroe than twice what that pack is designed to handle.


Regarding the capacitor bank, it may have to be quite large to help enough. Look around for a couple of recent threads on Ultracap / supercap usage; the math was posted on how to figure out what you would need for capacitance vs power usage vs time.

 

[akumar55]

Thank you both for your help so far. I did look around around the forum and kind of where I got the idea from. However, I was stumped while trying to figure out the limits of the undocumented BMS in this pack and if I can rely on it during regular operation.

You're right that the 1200 watt figure is definitely greater than the listed capability of the pack. My thought going into it was that the pack would serve as the energy storage medium while I could use the super capacitors for transient power delivery. For this, I put together a set of super capacitors totaling 27.5V 0.8 Farads. I'm going to look for the thread you mentioned.

I'm just not sure if I need to add additional circuitry to protect the pack or if I can just hook it up and rely on the BMS to limit the power draw if I'm trying to draw too much current.

 

[amberwolf]

YOu'd have to test the BMS under load to find out what it does on overcurrent.

Probably it will cut out until the load is disconnected, then reset itself.

 

[999zip999]

The kit was 75.00 usd plus shipping ?

 

[akumar55]

The total was $125 -- $75 plus $50 for shipping.

 

[999zip999]

Some packs have a rating 800watts, 1,500 watts, 2,000 wattts. I wouldn't get a 1,200 rated pack I would get a 1,500 watt rated pack. Do run it at it's limit. The controller demands from the battery.

 

[dmwahl]

I have that exact same mower, and have measured startup current at around 100-120A. Normal current draw when cutting the grass is usually around 20-25A depending on how thick the grass it. Given it's a 20Ah pack with some fairly fat wires coming out of it, I'd guess that the cells themselves would do fine at 20-25A (1-1.25C). If the cells really have the internal resistance shown in the ebay photos, they will be fine at 25A. That said, I don't know if the circuitry attached could handle it. It will likely balance the cells, but if you can post a higher resolution photo of the circuit board so that any text is readable I would have a better idea.

I've found that the CMM1200 charger works great for LiFePO4 as well. It charges up to around 28V (3.5V/cell), then backs off to around 27.4V (~3.42V/cell). That's just about the perfect float voltage for LiFePO4. You'll be happy you made the conversion, mine has way more consistent power through thick spots and is much lighter than with the lead battery. I don't bother with a low voltage cutoff, and just use a 40A marine circuit breaker for overcurrent protection. To cut my entire 1/4 acre lawn takes about 12-15Ah depending on how long it is, so with a 20Ah pack I have enough reserve not to worry much.

 

[akumar55]

Wow thank, you've provided some awesome info here. Using the circuit breaker is a great idea. How often do you tend to trip the breaker? Also, how did you manage to measure the current for the mower?

I guess since you're using the CMM1200 charger and only charging the cells to 3.5V, you don't have any balancing circuitry built in? Since you don't have a low voltage cutoff, aren't you worried that the cells would discharge too low and get damaged? Or are you just able to tell from the power output that you're hitting the end of the discharge curve?

I'm going to try to take the thing apart and take a few pictures as soon as I get a chance.

 

[dmwahl]

How often do you tend to trip the breaker?

I've never tripped the breaker, it's the same 40A rating as the one built into the mower by B&D originally. Even during startup around 100A, it would still take several seconds for the breaker to trip which is much longer than it takes for startup. The original one that comes in the mower wears out after a while, but if yours is still working I'd just leave it in there. Looks like a little box in line with the positive wires, one stud is copper and the other looks to be tin plated. Fortunately they fail by tripping lower and lower, so you'll know if it's bad since you only get a few minutes of mowing at a time even with a full battery.

Also, how did you manage to measure the current for the mower?

I used an inline "Watts-Up" meter. It keeps track of total charge used, minimum voltage, peak current, power, etc. Quite a handy device to have around.

I guess since you're using the CMM1200 charger and only charging the cells to 3.5V, you don't have any balancing circuitry built in? Since you don't have a low voltage cutoff, aren't you worried that the cells would discharge too low and get damaged? Or are you just able to tell from the power output that you're hitting the end of the discharge curve?

I do have balancing circuitry built in, which continuously balances above ~3.40V (basically discharges all but the lowest cells to keep them together during charge). I've never gotten below about 25% charge, and typically I'm done mowing around 30-40%, but I know from how the old lead acid batteries performed that it starts to slow down substantially around 22V.

 

[dmwahl]

Some more data for you...

I had been charging my mower pack using the B&D charger for some time, and a couple weeks ago (right after my last post here) mowed the lawn to the point that I actually depleted the cells (7 out of 8 down to ~2.8-2.9V resting, the last around 2.0V). I hooked up my watts-up to measure current to recharge, and using only the B&D charger, put about 18Ah into the 20Ah pack. I then hooked up my lab supply set to 29.2V (3.65V/cell) and set current limit to 1A to top off the pack. After that it took an additional 2.5Ah before 7 out of 8 cells pretty much simultaneously reached 3.65V. Another 0.5Ah was needed to get the last cell up to 3.65V.

In other words, my 3 year old 20Ah headway pack (8S2P) has a capacity of ~20.5Ah, and the stock CMM1200 charger will get it to ~90% of capacity.

Once the pack was almost empty (I had about 1 minute of mowing I needed to finish) the mower speed started noticeably slowing down. That was around 2.8-2.9V per cell resting, which is still safe for the cells. I threw it on the charger for 10 minutes or so and finished, then took all my measurements.

 

[999zip999]

Good lifepo4 is is great, a little big and heavy. If run right many many cycles. Flat discharge line.