auxiliary (12v system )battery sizing.

[nathan.stowe]

Are there any guidelines for how big the 12v aux battery should be in an EV  (144 vdc pack & dc/dc converter).  I have power windows and a power convertible top.  I will see if I can make them manual, but lets go under the premise of a decent load for the motors.  It appears to me that the main pack will always have to "carry" the aux battery, so it really does not matter how much capacity the aux battery has as long as it can carry the peak current for the convertible top.  Right?

Thanks.

Nathan.

[matt.kenigson]

If you have a decent DC/DC and you perform regular maintenance any standard 12V battery should do.  Most of the time the power will be supplied by the DC/DC anyway.  The battery is there mostly in case the traction pack or DC/DC fail or fall below acceptable levels. 

[nathan.stowe]

Follow up:  I have used a 12 lawnmower battery for the 12v accessory system with a 30 amp dc converter.  This has worked well so far and it even runs the motors for the convertible top. 

There was a slight draw on the 12v system even when the car was off which ran the battery down a few times.  I have disconnected that circuit and things are fine now.  I am not sure if this would have been an issue for a full size battery or not.

[todd.dore]

I think there was a discussion during a meeting sometime in the last couple years about this.

I was getting about 0.5A draw on my 12V battery even with the car completely off.  It was running down my 12V and I couldn't figure out (initially) why my new battery was draining so badly, until I measured the current draw.

My solution was to switch the DC-DC converter to be on all the time, even with the car completely off.  Previously, I had it hooked up to a 12V relay which turned on with the ignition key.

That solved the problem.  The only issue now would be if I let the car sit for weeks without using it, or a couple of days with no juice in the main pack.  This won't happen so I don't have any problems.

My suggestion would be to connect your DC-DC so it is on all the time.  Others may have a different opinion.

By the way, how is that new brake pump working out?

-Todd

[rich.carroll]

I see several separate issues here;

Your aux. battery needs to be big enough to handle any accessories you plan, all at the same time.  It is conceivable that wipers, defrost, headlights, radio, and ventilation fans could all be on at the same time, so plan for such things.  The 12 volt aux battery is just a small buffer.  It does not need to be very big at all.

Most of the upper amperage DC/DC converters from Iota have the option of two outputs, changeable by removing a plug.  The plug only has two jumper wires in it, if you hook one of the wires to a relay, it is fairly easy to set it up so that the aux battery gets a  13.2 volt feed when the ignition switch is off, and 14.2 volts when the ignition switch is on.  This keeps the aux.  battery almost full, but gives nice crisp performance for turn signals and wipers, and nice bright lights for driving. 

You should determine if you have any SIGNIFICANT parasitic draws in your car's system.  The main computer (which stores information), the clock, the radio (to keep the stations and time memorized) all have a small parasitic draw.  You can fairly easily determine how much this is so you can feel secure in leaving the EV for a few days without intervention, and know it will be reasonably fully charged when you need it.

[todd.dore]

Rich -

Thanks for the heads up.  I've located the jumper wires on the Iota, and there actually are 2 wires - 1 black, and 1 green.  The voltage on the car without the jumper wire is 13.6V on all the time.  I could easily hook up a key-on relay to one of the wires to give me 14.2V from key on with the car running.  But, I don't know which wire to use - black or green? (if you happen to know).

Also, as far as a significant parasitic draw is concerned, I don't know if 0.5A qualifies as such.

[rich.carroll]

Leave the black wire without cutting into it.  Put the output of a relay to the green wire, and by switching the relay, you change the output of the Iota between 13.2 and 14.2 volts.

As to whether 0.5 amp qualifies as parasitic draw, yes.  Almost any small draw qualifies as a parasitic draw in my mind.  The question is really whether you can live with a time (hours, days, weeks, or months) with this draw without needing to do something about it. 

[ken.simmermon]

.02 to .05 amps is a normal draw with no lights or other accessories.

[rich.carroll]

I talked to John Emde when I got to the shop, and here is John's suggestion to clarify what I said:

Instructions for Dual Voltage jack on most of the Iota DC/DC converters we use

"Don't cut the black wire.  Cut the green wire and connect the two green wires to the normally open contacts of a small 12 volt DC relay.  (Do not connect the green wire to power, not to 12 volts, not to pack voltage.) Connect the black wire of the relay to chassis ground and connect the white wire to keyed 12 volts.  When the key is turned on, the relay energizes and connects the two green wires together.  The 12 volt battery will now be kept at about 13.2 volts when the key is off, and will go up to about 14.2 volts when the key is on.  The input of the converter must be connected to pack voltage at all times."  The quoted part is exactly what John had written up for our customers to use.

Some of the smallest Iota DC/DC converters do not have a dual voltage feature, and some of the very exotic ones have a triple voltage feature, but these instructions conver 99% of the Iotas used in most conversions.