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Charging problem

1736 Views 10 Replies 4 Participants Last post by  kennybobby
Hi All, four years ago I converted my Grasshopper lawnmower from gas to electric. I am using 15 Calb 180Ah Li-ion cells, an Elcon charger and CelanPowerAuto BMS v2 which I purchased from Lithium Storage. I also have a Link 10 battery monitor that I use to watch the Amphr's in and out. This has worked fairly well for the last four years with a few minor charging/discharge issues.

My current problem is that the charger is shutting off due to one cell getting to high voltage on the BMS before the charger has had a chance to EQ all the cells. I usually run the batteries to about 80-100 Amphrs discharge level and then recharge. With the BMS shutting the charger off early, the Link10 indicates there is still 15-20 Amphrs of charging left.

I thought the BMS was the problem as two of the cell modules have some corrosion on the chips due to condensation. I moved the BMS cell module and the same battery cell caused the BMS high voltage to shut the charger off.

I have tried to contact Lithium Storage and CleanPowerAuto to get a replacement BMS modules with no luck. I am concerned that if I continue to use the mower without fully charging all the cells I am eventually going to ruin the batteries.

Any thoughts on what I can do to correct this situation.

Thanks, Paul
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I am concerned that if I continue to use the mower without fully charging all the cells I am eventually going to ruin the batteries.

Any thoughts on what I can do to correct this situation.
OK relax, take a deep breath and step outside the lead acid box you are trapped in.

Last thing you never ever want to do is fully charge Lithium batteries. Unlike like lead acid that must be kept fully charge to minimize stress. Lithium is exactly opposite where they are stressed being fully charged. You will double the cycle life if you charge to 90%, and never go below 10%. This is called Partial State of Charge (SOC) and is where lithium Ion batteries perform best.

As for the one or two cells you are having problems with is telling you those cells have lost capacity and at end of life.

BMS system cannot balance an unbalanced pack. they can only maintain balance with very small differences in capacity.

As for Bottom Balance goes is exact opposite of what a BMS does called Top Balance. A BMS is used to fully charge batteries every time and is the last thing you want to do. Commercial EV manufactures do not allow the consumer to fully charge their batteries. It is the only way they can offer warranties. If they allowed the user to fully charge every time, they would go bankrupt with warranty claims.

OK Bottom Balance does exactly what it sounds like. You discharge the cells fully one time only when they arrive. You wire all the cells in parallel and walk away for a few hours for the cells to equalize. Makes no difference if you Top, Middle or Bottom Balance. If you Bottom Balance, after cells have had time equalize, you then discharge the cells fully until your final resting cell voltage is 2.5 volts. Once completed, install them in series and charge to roughly 3.4 volts per cell, about 90%.

In a Top Balanced system cell voltage is only equal at 100% SOC, but not capacity because no two cells have the same capacity. Bottom Balance on the other hand capacity is equal equal from charge up to fully discharge. When fully discharge all the cells = 2.5 volts. Example if you use 100 AH cells, the weakest may be 95 AH and strongest maybe 105 AH. The pack capacity is limited to the weakest cell. If you Bottom Balance to say 90% of the weakest cell of 95 AH capacity, charged up every cell in the pack will have 86 AH with 76 AH usable if you limit discharge to 10% SOC.
 

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Howdy Paul, welcome to the forum.

Do you have a handheld voltmeter with which you could measure the voltage of each cell? If so then make a list of the readings while at rest (with no recent charging or discharging event), and post it up here so we can see how they align.

Some TCCH/Elcon chargers seem to push the voltage too high, not sure if it is a calibration or software issue, but the result is chronic over-charging that in time causes cell imbalance such as you have experienced. Hopefully your BMS has limited the charging such that no permanent damage has occurred.

Depending upon your cell imbalance it may be possible to individually bleed off the high cells and resume operation with a modified procedure due to the defective module.
 

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Discussion Starter · #6 ·
Hi Kennybobby, each of the 15 cells reads 3.32 volts currently. Last charge was at 5 pm Monday. Hi voltage on the BMS is set at 3.7 volts, most of the other cells are about 3.35 right before the BMS shuts down the charger.

I usually disconnect one cell when I am not using the machine for more than two weeks. I usually have one cell that is a little lower voltage when it sits for more than a month.
 

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No need to worry about ruining cells by not charging to 100%, on the contrary, over-charging will definitely ruin cells.

The resting or open circuit voltage of LiFePO cells, when fully charged is about 3.33 V, possibly +/- some few mV depending upon slight chemistry differences, according to reports published by cell vendors, and measurements by users. So it appears that your cells are nearly full, i.e. 100% State of Charge.

To get to that state the charger must lead the voltage higher, e.g. 3.6 V, in order to create a potential difference for current to flow. The elcon procedure is usually CC/CV until current drops to some level, but that is where they tend to extend the final charge too long and over-charge.

Since you don't require an entire cell's energy in order to mow your yard, in the interest of cell longevity, it might be that you would want to limit your charging to 90-95% full SOC, e.g. 3.2 to 3.25 It will not ruin the cells, but extend their life and usefulness to not charge to 100%, plus there is always the danger of over-charging if something doesn't cut off properly due to a faulty resistor or capacitor.

Over-charging will ruin your cells--that has been discovered by many folks who have needlessly pushed to the upper limit. Give yourself some margin, both top and bottom, and they should last a long time. Obviously you have been doing something right to be on your 4th season.

Next time you mow, measure each cell OCV and write it down, and compare with the voltage reported by the BMS. Especially watch the problem-child cell to see if it is still higher than the rest. The BMS can be a good tool to help you monitor and care for your pack, but it needs supervision--trust but verify. The electronic components in the BMS are less reliable and more prone to failure than the cells, so you must be vigilant with a good voltmeter to ensure it is healthy.

In an ICE car you don't fill your tank to the brim-- trying to get those last few drops in can lead to sloshing out the spigot and wet pants. And you don't run your car to bare empty. Your cells are your energy tank, fill to 90%, run it down to 10%, and you will have long life from the cells.

If one or more cells is out of balance with the rest, then it can be either charged or discharged separately to make adjustment. Even a small phone charger can be used as a power supply to add charge, and a suitably sized power resistor can be used to bleed off excess charge. I'm not familiar with your BMS or Link 10 to know if they have such a feature built-in, but some do.
 

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Discussion Starter · #9 ·
This is a basic question that I should know the answer to, but I'm not sure. Do the individual cells need to be disconnected from series connections to get an accurate volt reading on each cell?

Sorry for the ignorance.
 

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Discussion Starter · #10 ·
Is there a better method than the Zantrex Link-10 for measuring the discharge while mowing? I watch the Amphr's discharged and charged and lately I'm always short on the charge side even though the individual cells read charged.
 

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... Do the individual cells need to be disconnected from series connections to get an accurate volt reading on each cell?
No they can be measured without disconnect. To get an accurate reading, it is better to have no significant current load drawing on the pack when making the measurement. BMS systems measure individual cell voltages without disconnection, and you can too.
 
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