Help with a power meter

[Wishes]

I have recently built 2 12volt 20AH lifepo4 batteries for a mobility scooter of a neighbor. I found some nice 20AH prismatic lifepo4 cells and combined 4 of them with a BMS. The mobility vehicle runs on 24 volts. So we have 2 of them connected in series, combined with a 24 volts lifepo4 charger. The setup has worked flawlessly so far.

I have one difficulty with the setup, the power level indicator that originally came with the mobility vehicle was designed for 2 x 12volt SLA batteries. So with the 4 cell lifepo4, which provides a slightly higher voltage, the power level indicator always reads full.

From what I was able to understand, and recently learned, I should be able to add a resistance to the positive of that meter, to lower the voltage level and get it to work within its proper voltage range.

What I do not know and hoping one of the many experts on this forum can point me in the right direction, is how to calculate the resistance value I need and do I need to put it on the positive, negative, or one on both?

SLA have a typical full charge voltage of around 12.8. As for the lifepo pack is at 14.4 volts on a full charge.

Wishes

 

[dogman dan]

I don't think that method will produce a useful enough meter. The problem is, lifepo4 will spend 70-80% of its discharge at approximately the same voltage. Adjusting the volts based meter for lifepo4 will mostly just give you a meter that reads at least half full, until the moment when there is just 5% left. Then it will show that, when it's pretty much too late. Better than nada, but you need to know were half is, not where 5% and 95% are.

The most effective thing you could do would be to add a wattmeter to the scooter. Then by looking at watthours or amp hours, and comparing that to the known real world usable capacity, you can tell exactly where you are on the discharge.

A Cycleanalyst is the deluxe wattmeter, but very affordable ones are out there on ebay for less than 20 bucks.

 

[Wishes]

I don't think that method will produce a useful enough meter. The problem is, lifepo4 will spend 70-80% of its discharge at approximately the same voltage. Adjusting the volts based meter for lifepo4 will mostly just give you a meter that reads at least half full, until the moment when there is just 5% left. Then it will show that, when it's pretty much too late. Better than nada, but you need to know were half is, not where 5% and 95% are.

The most effective thing you could do would be to add a wattmeter to the scooter. Then by looking at watthours or amp hours, and comparing that to the known real world usable capacity, you can tell exactly where you are on the discharge.

A Cycleanalyst is the deluxe wattmeter, but very affordable ones are out there on ebay for less than 20 bucks.

I had thought of that, but some indicator is better than none. Those mobility scooters have the motor, battery and controller all underneath the seat. I'd have to route the power between the batteries and the controller to the dash and back again. Not that those things pull many amps. I think this model has a 15 AMP controller. I have a few turnigy watt meters, the wiring on those is 12 gauge only i think. Not that they will over heat at just 15 amps, but i was concerned of some loss if I induced that much wiring on the power.

Am I concerned for nothing?

Wishes

 

[dogman dan]

Agreed, it will be a pita to run full power to the dash. Someplace, there is a thread on how to hack the cheap wattmeters to a remote shunt like a stand alone CA. It would be worth it in this case to make the modification.

But other than having an "Yes it did really charge" indicator, adjusting the lead meter doesn't do much for you. But,,, knowing the thing charged is pretty important.

You could use just a dash mount voltmeter for that though. It won't help much with SOC in the middle of the discharge, but it sure will confirm that a battery got fully charged.

 

[wesnewell]

Forget the stupid power meter and replace it with one of these.
http://www.ebay.com/itm/Waterproof-Digital-Voltmeter-DC-15V-To-120V-Red-Led-Voltage-Digital-Panel-Meter-/170846310527

 

[dmwahl]

Forget the stupid power meter and replace it with one of these.
http://www.ebay.com/itm/Waterproof-Digital-Voltmeter-DC-15V-To-120V-Red-Led-Voltage-Digital-Panel-Meter-/170846310527

That won't give a reliable indication of the remaining charge, for reasons mentioned above. Also, 15-20mA current draw is rather steep unless it's switched off when the scooter is turned off.

 

[wesnewell]

Sorry, forgot you're using lifepo4. You need a watt meter. This is one of the cheaper ones.
http://www.ebay.com/itm/Boat-RC-Helicopter-Watt-Meter-DC-60V-100A-Balance-Voltage-Battery-Power-Analyzer-/190880016407

 

[dogman dan]

The wattmeter would also need to be switched somehow, to not run draining the battery slightly overnight. But this would not be a problem if the scoot is routinely left on a charger when parked overnight.

 

[Wishes]

The wattmeter would also need to be switched somehow, to not run draining the battery slightly overnight. But this would not be a problem if the scoot is routinely left on a charger when parked overnight.

I would have to install a watt meter underneath the seat. You have to keep in mind, this is a mobility scooter for an elderly person. I don't think the watt meter underneath the seat would be visible to him while he is riding, he would have to get off to take a reading from it.

I think I will install both. I'll put a small voltmeter on the dash. I know it won't be that effective, but he will at least get a small warning that the batteries are close to their end. And with the Watt meter, he can take an accurate AH consumption reading by getting off the scooter.

In retrospect, i realize that I knew this characteristics of lifepo4 batteries but I got excited when i found some 20AH lifepo4 cells for under $20 and figured I could keep the cost low for him and decided to use those.

I think 6s lipo cells would have done a better job for this application, and even kept the stock dash voltmeter functioning properly with their 25 volts fully charge with a steady volt drop as the battery is discharged.

Wishes

 

[dogman dan]

He might find his range so much longer now, that he needs little more than a way to know the charger really worked overnight.

Tell him to keep on charging whenever convenient, don't go waiting for it to empty before recharging.