upgrading to 72 volts from 48 volts

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brownj24
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upgrading to 72 volts from 48 volts

I'm looking for advice on converting my bike to run off 72 volts. My current setup consists of a nine continents motor with a 35A controller running off a 48v 20AH LiFePO4 battery. Is the best way to get up to 72 volts to connect another 24 volt battery in series? If so, how is this done taking into account the battery management systems? Do I need to match the amp hours to my existing battery, or does that matter? Should I be concerned with the power or current being drawn damaging the batteries?

Has anyone upgraded from 48v to 72v that can share their experience?

Thanks!

amberwolf
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Re: upgrading to 72 volts from 48 volts

What is the rating of your controller? If it does not already work at 72 volts, it will need to be replaced or modified.

If you simply series your existing battery with a new one, they need to be the same Ah or else you will only get the Ah of the weaker one out of the system.

Theoretically, the current draw should be less on battery side with higher voltage, so that will actually help your batteries live longer. Assuming you intend to draw the same total power from them. If you want to still run the same amps thru them as before, and simply go faster with the higher voltage, then it would be the same as it is now for battery life (possibly less, depending on air resistance and such, since it is not a linear progression for power used to get faster--it can take a LOT more power to go only a LITTLE faster once you get to a certain point, different for different vehicles and aerodynamics).

If the main reason you're upvolting is to go faster, you might also seriously consider an aero shell for the bike, or at least partial fairings front and rear. They'll do way more for your efficiency than you would think.

--
Michael Elliott
Cybernetic Necromancers, Discorporated
Phoenix, AZ
Watch me build an electric-assisted recumbent bike from recycled junk:
http://electricle.blogspot.com

brownj24
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Re: upgrading to 72 volts from 48 volts

Thanks for the info! I'm not too concerned with getting much more top-end speed. I know the simulator at ebikes.ca does not account for wind resistance and other variables. My hope is that running at 72v will give me a little more speed, but also more torque and acceleration. I know the controller I have can accomodate 72 volts, so I guess it's just a matter of finding a 24v 20ah LiFePO4 battery to connect in series.

Are you suggesting that the upgrade is really not worth it? ie, you don't think there will be much of an appreciable difference in torque/speed?

Thanks

amberwolf
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Re: upgrading to 72 volts from 48 volts

Oh, I think you will probably get quite a bit of improvement, as I certainly did when I overvolted from 24V to 36V, a 33% increase, same as yours will be.

But I *also* changed my gearing to convert that extra speed down into more torque, which isn't something you can do on a hub motor like yours. ;)

Still, as long as your stuff all supports 72V, there will certainly be more total power available. Because of that, remember that your motor is only designed to dissipate a certain amount of power (as heat) in a certain amount of time, so if you run it at the same load for the same time you did before, it'll get hotter, and you should watch for problems because of it.

On the flip side, because there's more power available, it should take *less* time to get to speed or get past the load point, and so on average be the same temperatures, with the caveat that those temperatures will rise faster than before, and then fall sooner.

--
Michael Elliott
Cybernetic Necromancers, Discorporated
Phoenix, AZ
Watch me build an electric-assisted recumbent bike from recycled junk:
http://electricle.blogspot.com

brownj24
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Re: upgrading to 72 volts from 48 volts

Thanks for the info, Michael. I was taking a ride today and reviewing some emails back and forth from the guys at ebikes.ca and now I'm starting to think that something is up with my battery. I'm not sure what the resistance of the controller and motor are, but my cycle analyst is showing a voltage of 48 with power output of 900 watts max. I am pretty sure when I first started using the battery, I had the same voltage and power output of around 1300 watts max. Is it possible the battery management system on my LiFePO4 battery is limiting the output of the battery?

amberwolf
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Joined: Monday, December 31, 2007 - 03:19
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Re: upgrading to 72 volts from 48 volts

Thanks for the info, Michael. I was taking a ride today and reviewing some emails back and forth from the guys at ebikes.ca and now I'm starting to think that something is up with my battery. I'm not sure what the resistance of the controller and motor are, but my cycle analyst is showing a voltage of 48 with power output of 900 watts max. I am pretty sure when I first started using the battery, I had the same voltage and power output of around 1300 watts max. Is it possible the battery management system on my LiFePO4 battery is limiting the output of the battery?

Yes, and it could either be a fault in the BMS or a battery that is reaching LVC (low voltage cutoff) before it should, or dipping down below LVC and causing the BMS to restrict current from it.

Inside the pack there are wires from the BMS to the positive of every cell (or parallel set of them), so that it can sense what voltages each of them is at. If it is also a cell balancer (which is common), then it also uses those wires to "shunt" extra current from cells that reach full charge before others do, so that the full-first cells aren't damaged by overcharging them.

If one of those wires comes loose either at the BMS or cell end, it can cause problems of varying types, depending on what reading the BMS sees at that point. It is even possible for it to restrict the total amount of charge the whole pack is getting, if it thinks that a cell is charging way too fast or too much, so that it cannot fully charge the pack at all.

If your fully-charged voltage is still exactly the same as it used to be, then this isn't likely. However, it *is* possible for the pack to be cutting out early because it thinks a cell is getting too low too fast, either because it really is (a bad cell, or just way out of balance), or because the wire to the cell is broken or loose and the BMS reads it lower than it really is.

The only way to check either of those is to open the pack up and use a multimeter to test the individual cell voltages. That requires great care, as the energy levels you are dealing with can instantly vaporize the ends of metal tools and meter leads if they are shorted across cells (which can be quite easy to do inside some packs, depending on how they are laid out).

--
Michael Elliott
Cybernetic Necromancers, Discorporated
Phoenix, AZ
Watch me build an electric-assisted recumbent bike from recycled junk:
http://electricle.blogspot.com

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