selecting from two DC batteries

[PoorOwner]

I purchased two of these battery to run a motor, I would like to select the second one after it is depleted.
Can I use the switch in the second picture?

set up question, Is it safe to tie the negative of both batteries together?
There is a pigtail for charging on each so I will be charging both of them separately to full charge. But I am not sure if tying the negatives will cause any problems.

With the 1+2 switch, would this be a problem to switch to that setting, I think this is letting both battery work in parallel, if one battery is higher charge it should slow equalize. If doing this is too much problem I will get a switch that only has the 1 and 2 (without the 1+2)

 

[The Cobbler]

tying the negatives together will not cause issues.
that switch is designed to use either battery 1, or battery 2 , or both 1& 2 simultaneous ( in parallel)
I would not switch it to 1&2 when the 2 batteries are at different voltages. at least not make a habit of it.

 

[walta]

I Like what Cobbler said.

What I do not see in your photo scares me! That is, I do not see any fuses.

In the event of a short circuit, you must have fuse that will blow before so much current flows that the batteries explode or a wire gets red hot and starts a fire.

Ideally the fuse would be inside the battery or the first thing that connects to the battery.

Putting the batteries in parallel is risky. If one has more charge than the other it will try to recharge the weaker one and maybe so quickly that the battery get damaged.

Understand big lithium battery are not a joke they are a huge source of potential energy and treated with the same respect you would afford an equally sized container of gasoline.

Walta

 

[PoorOwner]

Thanks, I will add a fuse or breaker closest to each battery.

 

[dcg9381]

Thanks, I will add a fuse or breaker closest to each battery.

There may be a BMS in that thing. Often BMS's have overcurrent protection. But I still fuse anyway.

Putting the batteries in parallel is risky. If one has more charge than the other it will try to recharge the weaker one and maybe so quickly that the battery get damaged.

Understand big lithium battery are not a joke they are a huge source of potential energy and treated with the same respect you would afford an equally sized container of gasoline.

Is this Lipo or LiFe04? The latter doesn't have the fire hazard risk that is associated with "lithium". If they are not over-current protected, then yea, they can usually provide more power than a wire can take.

We run LiPo batteries in parallel all the time in the RC car/airplane/helicopter space... It's a pretty normal thing. But we disconnect 'em (a switch would be fine) when done.

Those are "T-plug" connection types, you can buy pre-made splitters and save yourself some solder hassle. Make sure you get some with the correct wire gauge.

The "overall" issue you will have eventually is "battery balance". If you charge them in parallel long term they will tend to get out of sync. To solve for this, you simple charge them individually every once in a while, so charging up stream of your A/B switch is fine.

 

[PoorOwner]

This is a Lithium Ion battery.
I am surprised how much lighter it is than LifePO4.

The only issue I had is this is 36V and I got a spark when trying to connect the XT60 to something, this actually char the connector a little each time. I got some XT90 spark arrest connector and will be using it.

 

[dcg9381]

The only issue I had is this is 36V and I got a spark when trying to connect the XT60 to something, this actually char the connector a little each time. I got some XT90 spark arrest connector and will be using it.

That's probably the fault of whatever device you're plugging the motor into... Likely it's a capacitor charging, setting up my Victron 48/12 converter, it did the same thing and it's pretty documented as "normal".

I like the XT90s better also.

 

[cgrutt]

I have the same switch on my boat but use it with large 12V AGM batteries not LIPOs. Shouldn't have any issues with tying the negatives together. I'm not sure about how charging will work out as far as balancing cells if you connect two together via the 1+2 switch. Just wanted to say it's quite a large switch, including the studs on back to make connections. It's meant for very heavy gauge wire and terminals.

 

[dcg9381]

Just wanted to say it's quite a large switch, including the studs on back to make connections. It's meant for very heavy gauge wire and terminals.

Yep, I had to order the right connectors for the terminals for use with smaller wires. I didn't have much luck finding an A/B switch in the 30A range. Mine is A/B only, not both.



With an A/B switch if you charge upstream, you're really only using one or the other. You can charge on A+B, but eventually you'll want to charge them individually to get all the cells to balance as best you can.

The battery itself should have an internal BMS (check on this) that balances the cells. Smaller Lipos have "balance connectors" for cells and special chargers.