Need help for using N-Mosfets to switch on either one battery or the other

[kilou]

Hi,

I'd like to add a second battery (36V Li-ion) to my ebike to increase its range. My goal is to use one battery at a time, not using both in parallel with e.g. ideal diodes or something. I'd like to keep it as simple and as safe as possible and just use a 2 way switch to choose which battery the motor will pull current from. My 36V ebike motor draws about 20-25 A max so I figured out it would be best to use 2 N-channel mosfets to control which battery is being used. The N channel mosfets I plan to use are rated for 80V, 150 A and have a resistance from drain to source (RDS) of only about 3 milliohms (see https://www.mouser.ch/datasheet/2/427/sup60020e-1595623.pdf). This means at 25A the mosfet will dissipate 1.875 W. This should be okay without heat sink because the mosfet can be used without heatsink up to 3.5 W at an ambient temperature of 35°C.

I'm a noob in electricity/electronics so I tried to educate myself about mosfets and came up with the following circuit:

The idea is that when the switch in the center of the diagram is in the I position, battery 1 will power the motor. on the other hand, when the switch is in the II position, battery 2 will power the motor. However battery 1 will always be on the bike so it would be used to control the gate on the mosfets. The two resistors R1 and R2 are used as a voltage divider in order to drop the input voltage of battery 1 from 36V down to about 10 V in order to power the gate of the 2 mosfets since these mosfets have a max voltage from gate to source (VGS) of 20 V.

Now I have a few questions about this circuit:

1) when the switch is e.g. in position I, the mosfet on the left will allow the motor to run from battery 1. However, this also closes the path from the positive terminal of battery 2 to the negative terminal of battery 1...meaning that the motor would also be running from battery 2 as well. Is that correct?? The same occurs when the switch is in position II. This allows battery 2 to run the motor but the positive terminal of battery 1 becomes connected to the negative terminal of battery 2 so that the motor would also be running from battery 1 at the same time. Does this mean there is a huge issue with my circuit?

2) What happens if the two batteries don't hold the same charge e.g. battery 1 is at 30 V and battery 2 is fully charged at 42 V? I want to use a switch to properly isolate each battery from the other (I don't want to run the 2 batteries in parallel with e.g. ideal diodes). Would this circuit allow any cross-current to flow from the battery with the highest charge to the battery with the lowest charge? In other words, is it safe?

3) Should I be concerned with voltage spikes and inrush current when switching either battery on? What should I do to prevent such inrush currents from damaging parts in the circuit (above all, I want to avoid any damage to occur on the Motor controller and battery BMS)

4) Any change recommended on that circuit?

Thanks a lot for your help!

 

[john61ct]

Just buy a BSS mechanical switch.

 

[kilou]

What is a bss switch? Most switch I've seen cannot hold 25A at more than 12VDC. Would you have a link?

 

[John in CR]

I can't imagine any reason not to parallel them. If it's to pull one off for charging, just do it with RC connectors, being careful to not ever connect them together if voltage is significantly different > 1V

 

[kilou]

...being careful to not ever connect them together if voltage is significantly different > 1V

That's exactly why I don't want to parallel them (+ battery 1 is 6 years old with lots of internal resistance while battery 2 would be brand new). I use this bike as a daily commuter and I need a 100% error proof system with the ability to use any battery without first having to check its charge. This is a commercial ebike (Bosch) and it needs to be started on battery 1 only (even if the two batteries are paralleled using ideal diodes) so I'd need a switch anyway because having to use RC connectors all the time is a pain. Anything wrong with my circuit?

 

[amberwolf]

Keep in mind that if battery 1 has a BMS that shuts down it's output when it runs low, it won't be able to power your switch.

Also, if you have two batteries taht are essentially paralleled, but you then (accidentally via circuit problem) connect the positive of one to the negative of the other, it'll cause a short across one of the batteries, and you could get a fire.


Another thought:

If the ideal diodes you don't want to use have the ability to switch on or off, you can use those as the switches.



One thing about using FETs as switches is that if anything goes wrong in them, their typical failure mode is a resistive on-state, leaving you no way to turn them off (other than physically disconnecting them), *and* the current that flows thru them (either direction) will cause heating within them (potentially quite a lot of heat).


It may be simpler to use a marine rotary-style Battery Selector Switch; they're generally rated for only 24v, but will liekly work fine for your purposes as long as you pick one capable of sufficient current.

I've been using a cheap (and cheaply made) Harbor Freight 12v battery cutoff switch for years for my SB Cruiser trike's 52v battery, and it's still working, switched on and off multiple times a day. I do hear a spark inside, but it's resistance hasn't increased noticeably so far. Eventually it will probably fail, but it's had at least a couple of thousand cycles so far (maybe twice that), and doing alright so far.

Using a good rotary style marine switch would probably last longer than the trike. (rotary because then the wipers swipe across the contacts, and help clean them, as opposed to relay-style contacts that just "tap" or "press" against each other, which are not self-cleaning and can stop working or weld together).

 

[John in CR]

...being careful to not ever connect them together if voltage is significantly different > 1V

That's exactly why I don't want to parallel them (+ battery 1 is 6 years old with lots of internal resistance while battery 2 would be brand new). I use this bike as a daily commuter and I need a 100% error proof system with the ability to use any battery without first having to check its charge. This is a commercial ebike (Bosch) and it needs to be started on battery 1 only (even if the two batteries are paralleled using ideal diodes) so I'd need a switch anyway because having to use RC connectors all the time is a pain. Anything wrong with my circuit?

The sag from high IR makes the old one able to deliver less capacity without being paralleled to the new one as well as kill it more quickly. Plus the only way to avoid failures or errors due to connecting batteries is to leave them on the bike all the time. I can't imagine wasting time pulling batteries off of my bikes all the time. On all my bikes the packs are mounted securely to the bike to keep them safe. The only connectors subject to wear and tear are the charge ports, and power mains are either bolted or soldered together.

I've got one bike quite similar to your situation that has one portion of 12 year old Makita toolpack cells that are sag city and won't run the bike by themselves but still have 50-60% of their original capacity paralleled with a string of much newer RC lipo. In parallel they give much more useful capacity than individually. Plus, since the Makitas won't hold a voltage over 4V/cell, they reduce the risk of the lipo as well as extend that string's useful life.

 

[john61ct]

Blue Sea Systems

Their lowest rated battery switches, the "mini" series (m-Series) can handle 300A continuously, 500A for 5 minutes, 775A for 1 minute and 1500A for 10 seconds

And that is the absolute smallest battery switch Blue Sea makes

 

[kilou]

Thanks for all your replies!! Ok then I'll consider a blue seas system battery switch. Much simpler than trying to do it myself with mosfets.

I'm also interested in the parallel setup while leaving the batteries on the bike. I never considered that but it may also be an option if everything is secured properly. Just not sure how that would work because currently my bosch battery cannot be charged on the bike and proprietary systems have some protections against DIY but I need to do some homework to see the possibilities. Might come back to you with further questions on that topic in the future.

Thanks!!