36/72V switch for Solar charging my bike

I get how to switch two 36V battery packs to output *either* 36V or 72V but I'm interested in a more nuanced use-case: I want to power the bike on 72V all the time, but when parked, throw a switch and plug in a 36V charge controller -- separate from the (now shut off) 72V main power -- to charge the batteries with sunlight.

Here's a diagram that explains my use case and demonstrates what I am missing:



Any assistance appreciated.

M

MJSfoto1956 said:

I get how to switch two 36V battery packs to output *either* 36V or 72V but I'm interested in a more nuanced use-case: I want to power the bike on 72V all the time, but when parked, throw a switch and plug in a 36V charge controller -- separate from the (now shut off) 72V main power -- to charge the batteries with sunlight.

Click to expand...

The diagram you drew on the left will do what you describe. If you don't like that, your other options are:

1) Do the same thing with a relay and use some clever wiring to switch it to 36V when the charger is plugged in. (usually needs an extra pin in the connector)

2) Use two isolated chargers, each charging a segment of the battery

3) Use a relay to switch the charger periodically between "high" and "low" sides of the battery (note - may not balance perfectly)

In all cases periodically switching back to 36V mode will balance the two sections.

The simplest method to do this that is oopsproof is to use a keyed jumper plug (that only fits one way) that connects the batteries in series to the controller for riding. When it is removed, neither battery is connected to anything (including it's counterpart).

Then the charger plug uses a connector identical to the jumper plug, but is wired to connect the batteries in parallel to the charger.

A side benefit is that if you needed an antitheft device, the jumper plug, when removed, would prevent the electrical system from operating.


I recommend using a connector that is relatively heavy duty and does not unplug easily, simply so you "can't" have connection issues with it while riding.

Personally, I'd use Anderson SB50 (or larger if you need really high currents; I use them for over 100A just fine), with teh housings bolted together top to bottom to make a keyed block, with a T-handle between them to pull it out with (it'll take a fair bit of force, so you want to bolt the battery-end-connector to the bike frame or heavy duty battery box wall).

You can also use the Anderson PP75s, they are the same contacts but in modular housings that can be setup to be keyed in different ways.

The biggest advantage to the Andersons is they have sacrificial tips so if you get a spark while connecting things up, it doesn't affect the actual contact surface. All of the ones I have are used off of other things (powerchairs, UPS equipment, etc), and still work fine (I reuse the contacts by keepng the existing crimped-in 8-10g wire in there, and connecting to that, usually).

The disadvantages to these (besides cost) is they are rather large for the current capability, require you leave enough wire out the back of the housing to let the contact float in the housing (or they won't mate correctly and will cause high resistance, with heating problems), and for best contact should be crimped to the wire; you may need special crimpers to do it if you don't have something of appropriate size already.

YOu could use bullet connectors of various types, but you'll have to work out a method for keying the connector so it cannot be plugged in wrong.

I have 2 36v packs that terminate into a Anderson plug that has 4 connector . I then have a matching 4 connector Anderson plug that series the two 36v packs to 72v that goes to the controller . I made a charge plug that plugs into the 36v packs that parallels the 2 packs to one . So when I what 72v the two plugs are connected . To charge unplug and plug charger cable to battery plug. This is simple setup that requires only the plug be connected for 72v , and un connected to charge.

bionx 1954 said:

I have 2 36v packs that terminate into a Anderson plug that has 4 connector . I then have a matching 4 connector Anderson plug that series the two 36v packs to 72v that goes to the controller . I made a charge plug that plugs into the 36v packs that parallels the 2 packs to one . So when I what 72v the two plugs are connected . To charge unplug and plug charger cable to battery plug. This is simple setup that requires only the plug be connected for 72v , and un connected to charge.

Click to expand...

Perfect!

M

I actually have the same problem but I am doing a gas generator (38cc 4-stroke motor). You can also split the output from each 36V battery into parrelel connectors and then hook one connection for the serial 72V and the other to the solar charger. Like this diagram. Confirm this though I am not the best with wiring.

bionx 1954 said:

I have 2 36v packs that terminate into a Anderson plug that has 4 connector . I then have a matching 4 connector Anderson plug that series the two 36v packs to 72v that goes to the controller . I made a charge plug that plugs into the 36v packs that parallels the 2 packs to one . So when I what 72v the two plugs are connected . To charge unplug and plug charger cable to battery plug. This is simple setup that requires only the plug be connected for 72v , and un connected to charge.

Click to expand...

That's exactly the method I posted just previous to your post.

bakaneko said:

I actually have the same problem but I am doing a gas generator (38cc 4-stroke motor). You can also split the output from each 36V battery into parrelel connectors and then hook one connection for the serial 72V and the other to the solar charger. Like this diagram. Confirm this though I am not the best with wiring.

Click to expand...

You can't have them in parallel and series at the same time; that will be a big fire unless you have fuses on the packs, in which case those will blow.


You would have to do what I and Bionx1954 described, or use some other switching method (switch, relay, etc), or manually unplugging everything and plugging it back in the opposite way each time (dangerous, because you can accidentally misconnect things).

Either way, you cannot connect the charger and the load at the same time to the battery, if they are in different voltage configurations. Only one at a time can be connected.

amberwolf said:

bakaneko said:

I actually have the same problem but I am doing a gas generator (38cc 4-stroke motor). You can also split the output from each 36V battery into parrelel connectors and then hook one connection for the serial 72V and the other to the solar charger. Like this diagram. Confirm this though I am not the best with wiring.

Click to expand...

You can't have them in parallel and series at the same time; that will be a big fire unless you have fuses on the packs, in which case those will blow.

Either way, you cannot connect the charger and the load at the same time to the battery, if they are in different voltage configurations. Only one at a time can be connected.

Click to expand...

LOL, thanks. Man this is not gonna be fun to constantly disconnect and reconnect. I guess I will just get a 72V charger...

amberwolf said:

You would have to do what I and Bionx1954 described, or use some other switching method (switch, relay, etc), or manually unplugging everything and plugging it back in the opposite way each time (dangerous, because you can accidentally misconnect things).

Click to expand...

In that case, can he just use a DC-DC step up converter and boost the voltage to 84V charging? Or use two of them if one voltage step up is too high. I know you lose some power in the conversion ~5-10% but maybe there are premium ones where the conversion power loss is closer to 3-5%, which would not be bad. And, then have the solar panels in parallel with the 72V battery pack?

Use 72v mppt or pwm deep cycle directly into bms

Or dc to dc converter if charge amps has a limit you need to follow