Thread: D4 - Direct Connection of Main to Secondary Battery

Nothing happens... the vehicle will just start up...

Won’t be a problem at all.

A very minimal reversal of flow will take place and that’s all.

Thanks Tombie. I will give it a go then.

I also found the related thread here Smart alternator and BMS

Originally Posted by Mike57

I have the VSR set up so that the second battery will be connected when the main battery reaches 12.7 volts and it disconnects when the main battery drops to 12.4 volts but with a 120 second delay.

Hi Mike and the settings are too low and too high.

When you are running around town, it is not uncommon for your cranking battery to settle to as low as 12.2v, and the Optima can settle to as much as 13.1v.

So you can end up with your isolator continually turning on and off in the wrong situations.

The Cut-In ( turn on ) voltage should be no lower than 13.1v and the Cut-Out ( turn off ) should be 12.5v to 12.7v to isolate the two batteries shortly after turning the motor off, or 12.0v to no higher than 12.2v if you want to keep the batteries continually connected with the motor turned off.

And you can set cut out voltage of VSR at12.0V,same as Traxide SC80.
This will give you more usable battery capacity,and the car will start no worries at all,at even slightly less than 12.0 VDC.

I think cut in of Traxide SC80 is around 13.5 from memory.


EDIT.....Tim just beat me.......

Originally Posted by drivesafe

Hi Mike and the settings are too low and too high.

When you are running around town, it is not uncommon for your cranking battery to settle to as low as 12.2v, and the Optima can settle to as much as 13.1v.

So you can end up with your isolator continually turning on and off in the wrong situations.

The Cut-In ( turn on ) voltage should be no lower than 13.1v and the Cut-Out ( turn off ) should be 12.5v to 12.7v to isolate the two batteries shortly after turning the motor off, or 12.0v to no higher than 12.2v if you want to keep the batteries continually connected with the motor turned off.

Thanks for calling that out. I will adjust the settings. I also just realised that when the batteries are directly connected the voltage seen by the isolator will be a combination of the main battery and the cranking battery. So that would mean it won't shut off until both batteries are below 12.7 volts if I use the higher setting. I guess that is a fully charged battery so it wont be a problem? Is that correct?

Should I also bother with the cut out delay of 2 minutes?

My main battery is also an AGM BTW.

Thanks for the inputs and I think have this clear in my head now. I thought I would post it up for information and in case Tim wants to pick me up on anything. (sorry I am an engineer and had to get this clear in my head before I went ahead).

The maximum charging current will be delivered when the alternator is at maximum voltage and this occurs when the vehicle BMS senses that the main cranking battery is low and the vehicle is being driven. For this reason it is good to partly use the main battery as well as the auxiliary battery to power your fridge etc. If some of the current for house appliances is drawn from the cranking battery down to say 50% to 70% charge then this will mean when the vehicle starts the alternator will deliver maximum voltage (and thus charge current) to both batteries once they are connected via the VSR. This will mean faster charging than the lower currents that would be delivered by a combination of the alternator running at a lower voltage and some current being supplied from the cranking battery if the cranking battery were to remain fully charged.

Now my next big concern was how to stop the high cranking currents from being drawn from the auxiliary battery. The answer to this of course is that as soon as you start to crank the engine the very high current draw will drag even a new and fully charged battery down to something around 11 volts (or certainly below 12 volts). As long as the VSR is set to turn off above 12 volts then during cranking the main and auxiliary batteries are not connected. Depending upon how quickly the VSR switches off there may be some initial current drain but only for a millisecond I am guessing which should do no harm. The turn off should also be instantaneous with no delay programmed in.
So this means the turn off voltage for the VSR should be set around 12 to 12.2 volts as Tim suggests so that it turns off when cranking takes place and allows the cranking battery to be partly drained but not near enough to stop it from starting the vehicle.

This method would also provide a faster charge than a DC to DC charger (the best of these will deliver 25 amps to the auxiliary battery) I am not sure what currents the alternator can deliver but I am guessing something approaching 30 to 40 amps to the auxiliary battery.

I suggest a 6 B&S cable be run to the rear of the vehicle for anyone like me with the auxiliary battery installed in the rear when needed. This is not so much for current handling but to minimise any voltage drop with the long cable run. I also suggest protecting that cable with 50 Amp circuit breakers at both ends.

I also note that the VSR that I have is more expensive than the SC80 that Tim sells and if I had known about Traxide years ago when I went down this path I would have purchased that system.

Hi again Mike and some good points there.

But I suggest you are going to have problems setting to low voltage cut-out level with no debounce ( time delay ) period.

This could cause oscillation in a number of situations.

Next, it is actually beneficial to allow the auxiliary battery to assist when starting the motor.

The auxiliary will be lucky to supply 10% of the starting energy required, but this is 10% less that has to be replaced in the cranking battery, which means a shorter drive time is required to recharge the cranking battery.

It also means a higher CCA is available when starting your motor and so on.

You get the picture!

Hi Tim

I tried to call you today but I will try again tomorrow because now you have just raised more questions for me to consider!

Mike