Best method and switch for multiple battery setup

tomjasz · Aug 31, 2015

My friend insists on a switching setup for his two batteries. One is a dolphin pack 48V 11AH frame mount, the other is a 48V 20Ah rack mount. (All Samsung 29E)
He wants me to help him wire in a switch so that when one or the other packs runs down he can "switch tanks" as one would in a vehicle with dual tanks. I haven't the experience to choose the proper switch or design the proper wiring layout.

Suggestions for which switch, and from where?
Wiring scheme?

Thanks.

Tom (and Mark)

He sent me this switch link
http://www.amazon.com/Toggle-Switch-DPDT-Position-On-Off-On/dp/B001PNMC16/ref=cm_sw_em_r_dptod_DDN3vb1DPKETK_tt

e-beach · Aug 31, 2015

tomjasz said:
My friend insists on a switching setup for his two batteries. One is a dolphin pack 48V 11AH frame mount, the other is a 48V 20Ah rack mount. (All Samsung 29E)
He wants me to help him wire in a switch so that when one or the other packs runs down he can "switch tanks" as one would in a vehicle with dual tanks. I haven't the experience to choose the proper switch or design the proper wiring layout.

Suggestions for which switch, and from where?
Wiring scheme?

Thanks.

Tom (and Mark)

He sent me this switch link
http://www.amazon.com/Toggle-Switch-DPDT-Position-On-Off-On/dp/B001PNMC16/ref=cm_sw_em_r_dptod_DDN3vb1DPKETK_tt

A double throw double pole switch could work. (on - off - on) Ground to center tap. Positive to the other taps. Got to find one that won't melt do to amps. It will be big.

Edit: Sorry that was an early morning comment.......The center post goes to the positive on the controller and the positive from one battery goes to one side while the positive of the other battery goes to the other side of the switch. The negative wire on the batteries goes to a common ground.

This one might work if you use both sides at the same time. It is similar to my setup, but I only pull 30amps max from my controller. And, I always keep a couple extras on the shelf because they do burn out about every 12 months or so.

http://www.amazon.com/250V-125V-Pos...35126&sr=8-1&keywords=on/off/on+toggle+switch

If you don't mind a Frankenstein look, a Knife Switch - Double Throw, Double Pole style will probably last longer.

tomjasz · Aug 31, 2015

Thanks, the one I linked is rated at 30A. His controller pulls 25A. If memory serves.

dnmun · Aug 31, 2015

if they are both 48v lifepo4 packs or 13S lipo packs you should use them in parallel continuously and not switch out one for another after discharging the first.

tomjasz · Aug 31, 2015

dnmun said:
if they are both 48v lifepo4 packs or 13S lipo packs you should use them in parallel continuously and not switch out one for another after discharging the first.

They are 48V 18650, one is 11aH the other is 20aH. I've been told that is not a good idea to parallel.

dnmun · Aug 31, 2015

you were just told wrong then, a very common occurance here.

if you have separate charging leads from the output leads then you have to isolate the two packs at the P- on the BMS or motor while charging them to prevent overcharging of one or the other.

when charging is complete and both are at full charge then reconnect at the P- spot.

if the BMS has a common lead for both charging and discharging then you can leave them connected all the time while charging.

show pictures of your packs if it is not clear to you.

liveforphysics · Aug 31, 2015

Running each pack individually means more energy wasted in voltage sag, than simply paralleling them to share current.

tomjasz · Aug 31, 2015

Thanks!

So confusing...

One is a dolphin pack the other a frame pack. Both have no access to the BMS -P, well other than opening them up...

dnmun · Aug 31, 2015

P- is the negative output of the battery. it goes to P- on the controller. if you don't wanna open the end to look then maybe the guy who sold it to you will send you a picture of the BMS.

dnmun · Aug 31, 2015

once you can get the packs balanced up to the same voltage on the chargers i think you could worry less about charging them in parallel.

it is usually the initial charging where the cells are most out of balance from sitting a long time and having only the formation charge. so that is where there is the risk of overcharging one of the cells.

once they get to the same voltage and stay there for awhile you could tie them in parallel and then continue charging using only the smallest of the two chargers. that would be safest in letting them balance together.

after that you could recharge with both chargers and if the smaller of the two chargers had a higher final voltage that would be best situation.

tomjasz · Aug 31, 2015

OK, here we go...

I've learned immensely for a future custom build. I have a second BMS coming with his order for the 20Ah battery pack for a future build of my own. Both of those batteries will have full access and I can use this information.

But, Mark just wants a switch. on off on. He'll charge individually. When one battery gets near the low end he wants to flip a switch to go to the backup battery. I'm cautious about the proper switch. We'd both like to know the safe switch to use and proper way to wire this switch.

tomjasz · Aug 31, 2015

I'm trying Mark. :lol:

markz · Aug 31, 2015

Household electrical breakers. Two of them. Turn one off, Turn other one on.
Those are AC so I dont know if they are good for DC.
Look for solar stores selling dc switches. They should have DC circuit breakers.

e-beach · Aug 31, 2015

There are no good 48v plus dc switches that I can find. If you find one, let us know. Just go with the one originally posted. It should do the trick. Just keep a couple of extras on the shelf for a back-up.

Edit:

Basic 2 battery wiring with double throw switch.JPG

tomjasz · Sep 1, 2015

So far we've come up with the attached. Mark called the supplier on Amazon and they suggested the Eaton switch.

Double-Pole — Rating 20 A at 125 Vac; 10 A at 277 Vac; 1-1/2 hp at 125 – 250 Vac; Recommended up to 15 A at 28 Vdc

http://www.farnell.com/datasheets/719441.pdf

EDIT

JUST FOUND...maybe this is it? Thoughts?

tomjasz · Sep 1, 2015

Another thought, How about a marine battery sector switch? Designed specifically for switching between 48V batteries.

*EDIT* NEVER MIND these are HUGE!

e-beach · Sep 1, 2015

I prefer the style with the lugs under the switch body. IMO they are easier to work on.

tomjasz · Sep 1, 2015

e-beach said:
I prefer the style with the lugs under the switch body. IMO they are easier to work on.

You wrote "lugs" but pictured screws? Do you mean crews are easier?

markz · Sep 1, 2015

Maybe he meant - screw on "lugs" where you twist wire around screw

tomjasz · Sep 1, 2015

markz said:
Maybe he meant - screw on "lugs" where you twist wire around screw

I should have realized that, but all the switches I found referenced "lugs" as the nomenclature for solder points. Some here like solder joints better. I've come to be one of those.

e-beach · Sep 1, 2015

He meant the one in the picture..... :roll:

tomjasz · Sep 2, 2015

Let me regurgitate, "I should have realized that".

:lol:

Reflector · Apr 27, 2016

I am considering the same option of switching for my two 48V 11Ah 'dolphin' batteries. Since these batteries have XT90-S connectors, it is going to be a right royal PITA trying to disconnect them from in the depths of my velomobile. I would prefer to use the batteries in parallel but have been advised against this by the manufacturer. If switching is my only other option (rather than plugging and unplugging) could two of these be the answer?

http://www.newfrog.com/p/photovoltaic-dc-circuit-breaker-125v-1-63a-for-solar-pv-equipment-74052.html?currency=AUD&gclid=CMTqnLXCsMwCFU9xvAodVBoCLg

ScooterMan101 · Apr 28, 2016

Perhaps this ...
... http://www.wirthco.com/position-mini-master-dual-battery-disconnect-switch-20393-p-1222-l-en.html

However why not just parallel the batteries when running the bike, Then , for charging unplug the parallel harness / or / just turn off each battery switch. then just charge each battery separately, using a timer. 1 hour each battery or 2 hours each battery, etc.
( I take note of how many amp hours I used for that ride and time the charging for each separate battery accordingly ) it should work even if you have 2 different batteries that have different amp hour capacities .

It would be more simple to to just parallel them , also by running them in parallel he can pull more amps , this is helpful if he has bought a generic Chinese battery pack like I did , that is only rated at 2c .

friendly1uk · Apr 28, 2016

I think people are missing the fact these batteries come off the bike. Isolating them with a considerable switch gap

That will be more than ample for charging one without the others involvement.

The switch ratings given are continuous load. The make current will be very high when switching a battery to a controller. Much greater than 20 amps. Some switches will be become pitted quite quickly then melt.

spdt seems the way, but watch what it is made of and don't use screw terminals unless your sure everything is fixed down solidly.

Best method and switch for multiple battery setup

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