Connecting 10S Li-ion and 12S LiFePO4 in parallel

Hi,

I have an older 10S Li-Ion pack, and a new 12S LiFePO4 battery that I'm currently using 1 at a time on an ebike. Both are 36V nominally, but the LiFePO4 battery has a higher voltage throughout its discharge cycle, and a larger capacity (about 12 amp-hours vs. 8 amp-hours).

That said, I'd like to use them in parallel - less voltage sag, and less hassle.

If I connect them with a diode on the positive (red) wire, positive end connected to the Li-Ion pack, will that work? I'll disconnect them to charge, and then only connect them when both are fully charged. The LiFePO4 should have a higher voltage pretty much all the time, and my reasoning is that all I need to do is make sure that it doesn't discharge into the li-ion battery.

The LiFePO4 can take a pretty serious charge current (1C at least). The Li-Ion not so much - maybe 6 amps?

Let me know what you think, and what diode in particular I should consider. It's an eZee forte bike, with a 25amp controller.

Thanks!

i actually put the diode between my lipo pack and my lifepo4 pack on the drains of the lifepo4 BMS. i do not put the diode on top of the battery at B+ of the lifepo4. so the diode points toward the P- spot on my BMS, the cathode end, so current can only flow from the lifepo4 to the lipo and not ever from lipo into lifepo4.

my packs are closer in voltage. 24S lifepo4 and 21S lipo and i charge to 87.6V usually and to 88V occasionally.

does that make sense?

for the diode i took 10 of the 10A15V schottky diodes i bot on ebay for cheap and soldered the legs together inside a ring terminals so i could mount the diodes on a bolt that clamped my ring terminals together to make the electrical connections of my batteries.

Thanks for the reply, dnmum. I think I'm going to get one of these:

http://www.ebay.com/itm/2-X-VISHAY-POWER-DIODE-70HF10-70-AMP-100V-4-BATTERY-CHARGERS-POWER-SUPPLIES-NEW-/371021668224?pt=LH_DefaultDomain_0&hash=item56629d5f80

And connect in the way I described earlier. I'll let ya'll know how it goes.

but you do not need a 100V diode. there is always less than a few volts difference between the packs so a 15V diode is preferable because the forward bias of the diode will be lower. so those are the wrong type of diodes to use. i understand that you do not know any semiconductor physics so it would not be obvious why a lower voltage diode is better to use so i will try to explain by saying that the doping of the semiconductor can be greater for a low voltage diode so that makes it easier for the current to flow through the diode when it is forward biased. you can look at data sheets and derive that information yourself too.

Thanks dnmum! Your explanation makes total sense.

However, if I use a 15V diode and for some reason the li-ion pack isn't connected (or shuts off due to low voltage), there would be a full 36V drop over the diode. Is that ok if there isn't any current flowing through it?

What do you think of these?

http://www.ebay.com/itm/VISHAY-GENERAL-SEMICONDUCTOR-1-5KE15A-E3-54-TVS-DIODE-1-5KW-15V-10-pieces-/171575399844?pt=LH_DefaultDomain_0&hash=item27f2b0d5a4

Or these?

http://www.ebay.com/itm/ON-SEMICONDUCTOR-SA12AG-14V-25A-TVS-diodes-QTY-10-US-Seller-/171562013448?pt=LH_DefaultDomain_0&hash=item27f1e49308

I ended up getting these:

http://www.amazon.com/20L15T-Single-Schottky-Rectifier-TO-220AC/dp/B008LTKBBA/ref=sr_1_3?ie=UTF8&qid=1418972529&sr=8-3&keywords=schottky+diode&pebp=1418972533953

Cheap and quick shipping on amazon. I'm wandering in the dark here - I don't know a TVS diode from a schottky from a power... but maybe they'll work.

I'll post back! Thanks.

i went and found the ones i bot 2012 and these are the same deal:

http://www.ebay.com/itm/20PCS-10AMP-Bypass-Blocking-Schottky-Diode-for-DIY-Solar-Cells-Panel-10SQ045-01C-/201244525226?pt=LH_DefaultDomain_0&hash=item2edb1bc2aa

i was able to make 2 of those 100A schottky diodes from 20 of these.

I ended up getting the diodes that I had linked to earlier. Supposedly, they're rated to 20 amps each, and my setup was unlikely to draw more than 10A from the li-ion battery. Still, I was concerned that the diode simply looked so small, so I used two.

I decided to attach them using JST-sm type female pins, crimping 2 of these pins per cable (4 in total, see the first pic) and then slide these crimped pins onto the diodes' pins (see second pic, sorry for the horrible quality!). I soldered them in place, and with a little heatshrink and electrical tape insulated them from each other. The final job looks good, and survived the first test ride just fine.





-Henry