I have a small electric vehicle for my son that has essentially two circuits 24V (for the motor controller/motors) and 12V circuit (lights, horn, and microcontroller). I have 2 12V - 22AH batteries and currently have a 100amp battery disconnect key switch between the 2 batteries with a 35amp breaker. The 24V circuit peaks at around 32Amps and about 15-25amps continuous. The 12V circuit is only 1-3 amps continuous (if lights are on or off they are high power led's using a Flex block 750mA constant current driver) and around 4-5 amps if he hits the horn. I currently have it wired like in the diagram below however on the ACC circuit when the 100Amp key switch is off I am getting a 12V reverse voltage on the ACC circuit. My assumption is it is back feeding through the Sabertooth motor controller somehow.

The question is what is the best solution to fix this because I know what I am doing right now is not good. I was thinking of getting a DC to DC step down converter but am hesitant because some cost $20 and others are $200 and the specs are close and I am not sure why. The other option I thought of was using a high current diode and just tap off the + and - of one battery so I would not get reverse voltages.

The other part that makes this more complicated other than the High currents is I have the disconnect in place between the 2 batteries for 2 reasons 1 to shut off power and 2 so I can charge the batteries in parallel using a 6A Schumacher 12V charger. I do this by having a 4 pin connector on the vehicle with one wire going to each battery terminal. Then on the Charger I have a 4 pin connector as well but the 2 positives are tied together and the 2 negatives are tied together thus putting the batteries in parallel when this is plugged in. I am doing this because I am drawing the 12V circuit off one battery so then putting the charging circuit in parallel should even the batteries back out during the charge/trickle charge and also I was hoping to not have to use two chargers or to have to charge each on separately.

This brings me to question 2, this charging method is fine and dandy and works as long as that key switch is off. If for some reason that switch was turned on while charging it would basically cause a direct short across both battery terminals. I know this and I always hide the key when I am charging it but my son does not and my fear is someday this could happen. With that in mind can you think of a way I could wire this using a 30Amp relay or switch or something that would make it so there is no way the two circuits could be complete at the same time? It would be nice if it worked automatically, meaning you plug in the charger and it disconnects the main supply line somehow. I know I could just put a DPST switch on their rated for 30A that in one direction is for charging and the other is for running mode.

On this same note I was reading some posts on here and SgtWookie recommended a Schottky diode on each battery terminal to prevent High currents from flowing from one battery to the other. I wanted to see if I should do that as well in my set up? Currently my charging wires are 16AWG and are roughly 2-3 feet long so they will not like anything more than say 10A.

I know that's a lot of info but any advice you can give me would be greatly appreciated. I know enough about electronics to get myself in trouble but am pretty green as far as most members on this form.

I did attach a basic wire diagram(GatorControlPower.jpg) one that shows my current setup base power wiring as well as my more detailed ones.

 

I didn't have time to read all. Only looked at first schematic.

Put separate fuse and switch on the 12volt line.

Use two 12V chargers.

Could also put a diode or relay in the 24V line, but I wouldn't.

To use a single 100 amp switch for all, put it in the low side.

edit:

Try small bites.
Others may be as lazy as me.

 

I do see your point about a separate switch but the current switch is a key switch so it is like starting a car which is what I am going for with this being a little car. I would like one switch to control everything, I can throw a fuse on there which I should have anyway but just have not got there yet.

I do also understand that I can use 2 12V chargers and that is an easy solutions but the battery's do not come out without lots of leg work. I really want this to be as simple as possible for the user of the vehicle which is why I would rather go complex electronics to meet my needs of a single charger circuit to charge 2 batteries that are normally wired in series.

My plan to make sure there are no direct shorts is to get a 40A NC relay that has a 12V coil. this will be in between the two batteries as well as the 100Amp disconnect. Then I will have a 6 pin charging port that has 2 wires to the relay coil and one wire to each battery terminal. The charger when plugged in will then bring all 3 positives and all 3 negatives into a parallel circuit. The part I do not understand is how dangerous this is if the batteries are out of sync, as this could have huge amp draws from one battery to the other.

Hopefully that all makes sense.

 

A blocking diode in the acc circuit should block reverse voltage.

Connecting batteries of different charge states isn't a big concern.

Series parallel switches are tricky to make foolproof.

Why remove batteries? You can have two charger ports. One for each battery.

 

Is that preferred to say just having a 24V circuit with a DC to DC converter that steps the voltage down to 12V?

 

Just a choice.
Diode is simple.

With a converter, you can have a straight 24v system. Charging at 24v.

Or get 24v acc.s.

Sorry, I added some edits while you were posting.

 

I do agree I should have just got a 24V charger but I have the 12V one at this point. I think I will go the route of having the 24V circuit and step down the ACC circuit. The one question I have is what gauge wire should I have for the Parallel charge wires? Currently I have 16Gauge stranded wire which should handle the 6amp charger but will it handle battery to battery if they try to equalize? I have yet to plug the charger in just yet for fear of melting those wires or wrecking a battery.

Thanks for all the input by the way!!

 

As I said, series parallel is not easy to do.

I'm pretty handy with relay logic and haven't got it yet.

Show a schematic of charging circuit before testing.

A DPDT relay could work, but it must have forced contacts on the same armature. At a minimum!

It may require another master control relay and interlocks to be safe.

The short #16 wire isn't a big concern. (least of my worries) Fuse it at 10-15 amps to be safe.

Sometimes the best solution is not what you want to hear. I'd get a second 12V charger!

 

I was thinking something along these lines so when charger is connected on + and - are set in parallel and it kills the rely so the series circuit cannot be completed.

 

The first thing I notice is 12 volt power to both controllers when charger is plugged in.

 

Great catch I was trying to throw out a quick diagram before lunch and I missed that. So I guess I could do a DPST or DPDT (not really needed) relay or do 2 SPST relays like in this corrected diagram.

 

I did mention that it's not as easy as you might think.
Any way to talk you out of it?

Seems your a lot like me! You like the challenge.

Latest diagram has timing issues.

It takes time for relay to pull in.

Trace it out, charger connected, but relay not pulled in.

This is the condition momentarily and also relay failure condition.
Not good!

Also, smart chargers do not put out without feedback from a good battery.

I'm sure all this can be resolved, but I'm not sure it's worth the added complexity. KISS

 

This wasn't clear when I looked at it again.

The charger plug will short the batteries if the 100A switch is on while inserting.

It will happen sometime!

The other issue is minor. Momentary 12V power to load while relay pulls in.

 

I did mention that it's not as easy as you might think.
Any way to talk you out of it?

Seems your a lot like me! You like the challenge.

Latest diagram has timing issues.

It takes time for relay to pull in.

Trace it out, charger connected, but relay not pulled in.

This is the condition momentarily and also relay failure condition.
Not good!

Also, smart chargers do not put out without feedback from a good battery.

I'm sure all this can be resolved, but I'm not sure it's worth the added complexity. KISS

I hear you but this is why I would put the relays in, the charger will have a direct feed to the terminals. the 100amp switch will always be off when I plug the charger in. The relays are more of a backup in case he tries to turn the car on while it is charging. I mean I am open for suggestions on another way if there is a better one. If you are really saying that I should not even try this approach and think I am in for trouble no matter what I will back down and just charge each battery one at a time. However I would prefer to figure out a safe way to do this regardless of the complexity I just want to make sure I do it right

 

This wasn't clear when I looked at it again.

The charger plug will short the batteries if the 100A switch is on while inserting.

It will happen sometime!

The other issue is minor. Momentary 12V power to load while relay pulls in.

Yes this is the whole reason I posted on this forum is because I noticed that and it scared me a bit. I think the relays in the new diagram help add a second level of protection though correct. I mean if the key is on and you plug in the charger yes you have a direct short but hopefully only for a few milliseconds (I know this is not ideal). This should never happen since I am the only one who charges it, my fear is him trying to start it while it is charging. That I could see happening.

 

My opinion?

It will happen!

Edit,

As I can't talk you out of it. Fuse the leads between charger port and relays.
They need to be fused, to protect them, in any case.

It is a unique idea. Does charger supply power to relay coils with no feedback from batteries? (dumb charger)

 

My opinion?

It will happen!

Edit,

As I can't talk you out of it. Fuse the leads between charger port and relays.
They need to be fused, to protect them, in any case.

It is a unique idea. Does charger supply power to relay coils with no feedback from batteries? (dumb charger)

Well I was thinking since it would parallel the batteries with the charger that in theory the batteries would power the coils. It is a Smart charger so you are correct in saying it will not power the coils right away anyway. Will all that said how would you do this?

 

That sounds right!

Then the only issue I can see is when 100 amp switch is left on.

One more relay powered by the 100 amp switch may fix that.

 

Then all my issues are covers except for the ones I did not think of yet

 

Then all my issues are covers except for the ones I did not think of yet

Plus the ones I haven't seen.

Don't forget the fuses before testing.