Bank Charging or PowerCheqs?

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djkees
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Points: 29

After some lurking and shopping around, I am finally buying my Zapino (great deals in San Jose right now) I have read many posts about bank charging and BattEQ / PowerCheq. I assume you only need one or the other. Which is better? The bank charging system sounds cheaper. However, the PowerCheq system sounds more robust, since it balances charging and discharging of the batteries.

I'd love to hear your thoughts on the subject.

Thanks for all the great info. I will try to post my experiences with my new scooter.

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chas_stevenson
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Re: Bank Charging or PowerCheqs?

I have done both.

PowerCheqs - These units do indeed work well, they work on both charge and discharge. They help increase range and maintain power more evenly over the discharge cycle.

    PowerCheq Problems

  • When the batteries are stored they continue to try to keep them balanced. So when one battery in the string drops .5 volts below any of the other batteries in the string due to self discharge the PowerCheqs take power from the higher battery and charge the lower battery so they match, just as they are designed to do. This process however continues until all batteries in the string have been depleted, not good.
  • If a battery pack has been through the above process and you connect the charger to try to bring them back to life the PowerCheqs interfere with the charge's ability to know when the batteries are at full charge and will continue to charge them until they cook, again not good.

Bank Charging - This is the use of multiple charges to charge a string of batteries in a pack. This method balances during charging only. This method also helps increase range and maintains good power output through the discharge cycle.

    Bank Charging Problems

  • The chargers selected for this task MUST be isolated electrically. There is a easy way to check for isolation. Using a meter set to Ohms check the resistance between the input and output wires of the charger. Be sure to check all possibilities including the ground, or third connection, on the input plug. If the charger is isolated there should be NO connection from any of the input to any of the output wires.
  • Each Charger may have a different cutoff voltage even if the same brand and model. If possible it is best to open the units and adjust them so they are all equal. If this is not possible then you may need to purchase extra charges to get them all close and return the ones that don't meet your requirements. (That is what I did. More on that later.)

Well I guess I still haven't answered you question. In my opinion Bank Charging is the better of the choice and here are my reasons.

  1. Bank Charging is great for keeping the batteries balanced and very easy to implement. It also helps both range and power output of the battery pack.
  2. PowerCheqs help hold the power output at a more constant rate during the discharge process than Bank Charging.
  3. PowerCheqs help the range by a very small amount.

Referring to items 2 and 3 I must elaborate. The difference in power output is only noticed for about the last mile or 1.6 kilometers of range. The difference in range is about .25 miles or .4 kilometers.

One other difference I have found is Bank Charging gives me longer life of my SLAs which equates to more charge cycles.

I feel Bank Charging is the better choice. What I did was purchase 3 charges for my 36-volt string. I checked each charger during the charge process and kept the 2 charges which were closest in voltage. I took the other charger back to the store and told them it did not cutoff at the correct voltage and exchanged it for another unit. It took 3 trips to get 3 chargers that are all equal, well with .4 volts, and my pack seems to work great.

Grandpa Chas S.

PJD
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Joined: 11/22/2006
Points: 1411
Re: Bank Charging or PowerCheqs?

As a response to the above, I used powerchheqs when I still had SLA's in my e-max and didn't have the problems Chas listed above. Storing my scooters over winter with the powercheqs connectes led to no noticeable self-discharge. Note that you should be doing a top-off charge every few weeks when the scooter is in storage anyway.

The main issue with powercheqs is that, with SLA's, they are only balancing, or trying to balance, the 12-volt batteries. But the six individual cells in the batteries get out of balance. The powercheqs can do nothing about that, or they may even aggravate the problem in a manner like Chas describes.

This is a problem peculiar to SLAs. The individual cells in a flooded lead acid can be equalization-charged through a deliberate overcharge, because there is abundant liquid electolyte (H2SO4), and electrolye can alway be topped off. SLA's rely on a just the small amount of acid-soaked fiber glass matting or (for silicone batteries), acid in a silicone jelly.

I've become convinced that using SLA's in EV application is just not economical if you are going to be using the scooter daily in the warm season like many of us do. You may only get a season and a half out of a pack. If the application requires spill proof batteries, like scooters, I believe Thundersky or similar LiFePO4's are the only way to go. The up front cost is much higher, but they will more than pay for themselves. And, their performance is superior.

The problem is, they need a battery management system (BMS) that keeps individual cells from getting overcharged or over discharges. No currently available electric LiFePO4 scooters have these, and most dealers I've spoken to are not very well informed about this issue. The makers of some BMS's out there are aware of their need, but not well informed of the rather simple BMS requirements for e-scooters, so they tend to only produce elaborate and very expensive BMS laden with datalogging and control features that we don't need. Right now, the only available BMS for our needs is being sold in a do-it-yourself kit form here:
http://www.tppacks.com/products.asp?cat=26

garygid
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Joined: 12/19/2008
Points: 441
Re: Bank Charging or PowerCheqs?

Sorry, string (series) charging 3 cells at 6 amps does NOT divide the current between the cells (parallel charging would tend to do that).

In series (string) charging, all of the current goes through every cell. So, a 6-amp "pack" charger will charge the battery twice as fast as a "bank" of 3-amp one-cell chargers.

However, accurate "bank" charging can be great at keeping cells in balance.

__________________

Cheers, Gary
XM-5000Li, wired for cell voltage measuring and logging.

chas_stevenson
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Re: Bank Charging or PowerCheqs?

Garygid,

Good catch I was wrong about that. Thanks for keeping the facts straight. I confused parallel with series. I have removed the incorrect information garygid found in that post.

Thanks,
Grandpa Chas S.

djkees
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Joined: 03/06/2009
Points: 29
Re: Bank Charging or PowerCheqs?

Thank you guys very much for the info. I picked up my Zapino yesterday and am having a blast. I've gone on a couple small rides (about 9km total) and have put the charger on once. I start my daily commute (9 miles round trip) with the scooter tomorrow. I am going to have to use the string charger for now, but I plan to start building a bank charger ASAP. It sounds like that will be my best option for now. Any suggestions on chargers to use?

I hope there are better LiFePO4 options ready when it's time to replace the pack.

Thanks Again, Dan

andrew
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Joined: 11/28/2006
Points: 1361
Re: Bank Charging or PowerCheqs?

I have not purchased one, but the BattEQ looks much easier to install than the powercheq units. I tried using powercheqs on my motorcycle, and I had problems with fuses blowing on the powercheqs. Also, they have a kind of large voltage variation necessary to begin equalizing. This could mean a significant voltage difference between opposite ends of a pack.

Another down side to bank charging is that if one charger goes bad without any indication, then you may undercharge a battery and overdischarge it. Secondly, this emphasizes the point that bank charging is complex with lots of chargers (and components within them) to fail. Once the weakest component fails, then you may have problems unless you are monitoring the situation.

Of course, with any system it is wise to monitor how things are going every once in a while. A packtrackr may be a bit overkill (though very useful), but at least measure the battery voltages every few weeks if possible, or check the output on all of the chargers with meters or LEDs, or check the charger's display screens.

Anyway, in conclusion, I would go with a BattEQ but you can see that this is personal choice with unique advantages/disatvantages, and I have not thoroughly researched what problems can arise with one.

andrew
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Points: 1361
Re: Bank Charging or PowerCheqs?

PJD wrote:

The main issue with powercheqs is that, with SLA's, they are only balancing, or trying to balance, the 12-volt batteries. But the six individual cells in the batteries get out of balance. The powercheqs can do nothing about that, or they may even aggravate the problem in a manner like Chas describes.

This is a problem peculiar to SLAs. The individual cells in a flooded lead acid can be equalization-charged through a deliberate overcharge, because there is abundant liquid electolyte (H2SO4), and electrolye can alway be topped off. SLA's rely on a just the small amount of acid-soaked fiber glass matting or (for silicone batteries), acid in a silicone jelly.

I've become convinced that using SLA's in EV application is just not economical if you are going to be using the scooter daily in the warm season like many of us do. You may only get a season and a half out of a pack.

While it is true that powercheqs, or bank charging can not directly balance the 6 cells within a 12v battery, this does not mean that charging the battery per the manufacturer specifications won't balance the cells. This is because 14.5-15v (typical recommend charge constant voltage limit for 12v battery for cycling purposes) will overcharge the cells and cause gasification. It is over the recommend float/indefinite charge voltage limit of ~13.5-13.8v. Fortunately, the overcharge time is short if the charger is working properly and the battery can recombine nearly all of the hydrogen/oxygen generated back into H20 within the battery.

Lead-acid cells have a great advantage in how they charge. They will stop absorbing charge once they completely charge. This made them great initially for EVs 100yrs ago where they didn't have any sophisticated chargers, but just dynamos to generate electricity. This means that if the voltage is held constant, then the cell's resistance to current rises high at end of charge, and the current will drop very low BUT they will draw more and more current if overcharged. If the battery is charged at a constant voltage of 14.5-15v/cell at 20 degrees C, and held at that voltage until the current drops to .01CA (or .4 amps for a 40ah battery), then I am certain that the cells within the battery will be equalized. 6 cells appears to be a practical limit for equalizing in this manner,i.e., just 6 cells held at 2.45-2.5 vpc. If more cells, then a higher overcharge voltage may be necessary to force a higher overcharge current and equalize the cells in a practical time limit, i.e., overcharge the cells faster to ensure that they are all charged to at least 100%. This is why it is practical to equalize a larger string of 6+ cells in a flooded cell pack with a higher overcharge voltage.

I'm sorry if this sounds ridiculously complicated. To try and simplify it, consider a cell and how it interacts dynamically with another cell in a 2-cell lead-acid battery.

Cell 1 (C1) gets charged sooner than Cell 2 (C2). At end-of-charge, C1's resistance to current drops lower and lower, and hence it's voltage drop, or voltage across the terminals rises. Eventually it is fully charged, but the charge current through both cells must be equal in a series circuit. So, C1 now begins to be overcharged, and once the voltage passes a threshold, it's resistance rapidly falls, i.e., it starts to draw more current. This prevents C1's voltage rising infinitely and allows C2 to be charged. This is how forcing an overcharge will balance both cells.

That is how they charge the batteries (just 12v (6 cells) 14.5v constant until current drops to .01CA) when they do laboratory testing of cycle life, and how they come up with the 300 cycles at 100% DOD. If these conditions are imitated, then this kind of cycle life could be expected. The question is, are these conditions being met? Some chargers cut off the charge current at much higher than .01CA, or don't hold the voltage constant at 14.5v, but rather cut the voltage back to float voltage once the charge current drops below max, i.e., once the battery hits 14.5v. While that's not a bad charging scheme, I wouldn't do it in a cycle application, and the Soneil's do this! Also, charging a string of more than one 12v battery is certain no not meet these conditions as we know, one battery will be at 16v while another is at 14v after a while when the string charger cuts off.

For the Zapino range of 20 miles/charge (very conservative?), then that's 6,000 miles. That is not great, but I think it's at least practical considering the battery replacement cost.

LiFePo4 are still new, and there doesn't appear to be any of the more main-stream high-quality battery manufacturers selling large cells directly to consumers yet, e.g., panasonic, sanyo, SAFT, ect. You can get chinese cells, which are very expensive. I'm not saying they are not practical, or a suitable choice; this is just a personal opinion. I would rather go with a high-quality NiMH or NiCad cells at this point than chinese LiFePo4 if I was to do anything beyond lead-acid. NiMH or NiCad's cycle life degrades gradually, and a large enough pack could get a very long cycle life provided the cells are charged at .1C even without balancing.

KZ750 Motorcycle Conversion
900 watt scooter
Pic from http://www.electricmotorbike.org/

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