[frodus]

Say you're using a 120V pack, and you need 12V... ignore losses for a second... thats a 1:10 voltage ratio. Say you want 10A on the 12V side. Current and Voltage are inversely proportional with the same load, so 10A on the output side is 1A on the input side. 1A is Nothing when draining your pack.

Now, switch to a 12V battery. You're using 10A out of the battery. Say you run for an hour, and you've got a 20Ah battery... it could run 1.5 hrs just fine (to 75% DOD). It'l take a couple hours to charge that back up to what it needs to be, and it'l take another charger to charge it. Sure there are minimal losses with JUST a 12V battery, but it'l take longer to charge back up.

With the first option of taking it off the pack, you can charge 120V worth of bats at the same time and give the pack back the lost charge in very little time. 1A isn't much. Even 2A input/20A output isn't much. Its not like you're sucking 20A off the pack, just a couple Amps. Not really enough to do anything to cause you to get less range.

Another thing to consider.... everyone says "lead=range"... so why not put that "extra battery" to use in the HV pack used for the drivetrain.... and have it put to good use.... After all, another battery increases voltage (speed) and range. Your gains in range by putting in another battery and putting it in with the pack would be better than having it seperate and running the accessories off it.

Lots of DC converters are above 85% efficient... minimal losses (we're talking tenths of amps. Thats nothing on a 120V pack.

Another thing to consider, if its a 20Ah battery running accessories, and you're discharging it at 10A of its 20A rate, the discharge curve might be steeper due to the peukert effect. Its not linear. The more you draw, the less capacity it has. You'd be better off drawing only an amp or so off the HV pack, than you would 10A off one battery.

[Qer]

Originally Posted by favguy

At the risk of bieng naive, why do we need DC to DC converters at all?

Well, in the end all electricity spent during the trip must be stored in led that has to be hauled around. This is the pros and cons as I see it:

Having an auxiliary battery:
+ Won't draw power from the main battery pack.
+ Dirt simple, low failure rate.
- Must be over dimensioned to not run out before the main pack in case of queue, ie dead weight.

Having a DC/DC:
+ Higher voltage, better light.
+ Will most probably increase range if the same total weight of led is kept.
- Must still have a small battery when DC/DC is off line.

So in the end I think it's up to what you prefer. If you, for example, can't have higher voltage than 144 Volt in the pack and it fits easily in your car you don't gain anything (except from a higher voltage) by excluding the auxiliary battery. If you on the other hand can increase the main pack with another battery if you don't need the auxiliary battery, for example because space is cramped, it can increase both top speed (higher voltage) and range (higher total Wh).

Personally the increased voltage is a reason enough for me, but that's my choice and doesn't mean that I'm Right! YMMV.

[lazzer408]

Originally Posted by DVR

Brilliant idea!!
I must confess tho, that while I'm fairly handy with a soldering iron, and am pretty adept at following instructions like the ones above, I cant follow the tech talk it's like a foreign language

If you do these mods as described here, are you saying that these will do the job of a $300 DC/DC converter?
What would the max output in amps be without stressing the thing?

Find a friend that can help you thru it. It's as simple as putting your pack voltage right to the ac plug to make it work. If you want better efficiency then put your pack voltage to the + and - terminals of the bridge rectifier, as shown, and snip out the rectifier. If you want more voltage output you can add the 27k resistor as shown for 14.2v. That's it. These mods are only for a 250w.

The 250w might be a little on the skimpy side for a car. I chose that because I only have a motorcycle plus I had it kicking around. I guess if you had all LED lighting and didn't plan on a huge stereo you might get away with using the 250w. Headlights alone can draw 10-12 amps. Thermaltake rated the 250w with a 21-amp max load.

[lazzer408]

Here's another DC-DC. It's a Vector Travel Mate 12v 6a adaptor. I bought 3 of these for $7ea. Here's one powering a 55v HID headlamp. Of 120vdc. It's only 72 watts but might be useful for some sort of auxiliary/dedicated power source for a 12v device.

Output 12v 6a
Input 60-144v (roughly)

I have the black and yellow input wires soldered directly to the main capacitor. Yellow is positive.











The HID above is drawing .56a from the 120vdc batterie pack. You can't see the pack in the picture but it's 10 7ah scooter batteries.

[favguy]

Thanks for the input guys, maybe I should rethink this hmm..

On to the converter, is the 450 or 650 watt model likely to have the same circuit board?

If you use one of these with a small capacity battery to give lights radio etc if parked up with main power off, do you think it's safe to leave it switched on all the time you're running, ie, giving a constant charge to the battery when input current exceeds output current pull, which with the bigger units should be the case

[lazzer408]

Originally Posted by favguy

Thanks for the input guys, maybe I should rethink this hmm..

On to the converter, is the 450 or 650 watt model likely to have the same circuit board?

I doubt it. I'm sure the circuit design will be -similar-. It's a boost convertor (for power factor correction) followed by a push-pull step down converter. There's only so many ways to skin a cat. Send it to me I'll tweak it up to ~14v and post the info for everyone else. I'll send your modified unit back to you ofcorse. You pay shipping.

Originally Posted by favguy

If you use one of these with a small capacity battery to give lights radio etc if parked up with main power off, do you think it's safe to leave it switched on all the time you're running, ie, giving a constant charge to the battery when input current exceeds output current pull, which with the bigger units should be the case

I'm not sure I follow you. The input current will never exceed the output current on these converters. The most you'd ever see on the input side is about ~3-4 amps. It should be ok to leave the converter on but I wouldn't leave it unattended. The voltage output is regulated. If a battery is connected, it will charge if you've done the volt-mod for 14v. If the output current exceeds it's current limit the converter will shut down. I believe it's self resetting and will power on again reguardless if the excessive load has been removed or not. It will continue that cycle until the load is removed or it burns up.

[favguy]

This 600 watt unit looks like it might be a bargain, if one of you electronic guru types can tweak up the output voltage a little

http://cgi.ebay.com/New-PowerStar-60...QQcmdZViewItem

[lazzer408]

Update:

Vector 6a DC-DC 14.3v mod. Easy enough. 15k in this one. Any of these converters can be volt modded.

[frodus]

I think he was asking "do you keep it on when you're in drive mode"

The answer is "Sure". It will charge the battery if the bat voltage is lower than the power supply voltage. Usually in cars/trucks/motorcycles, the DC-DC converter is wired to turn on when the key is in the ignition and turned to ON.

I don't know why you'd need another battery though, maybe just to smooth out the current when you initially turn lights on... to handle the inrush current. Otherwise, the DC-DC converter should be sized to handle the load anyway. Its not a bad idea... but you'd likely only need a 20Ah battery to smooth things out.

It just won't work when you're "off" and not in drive mode, but the bat will allow you to use accessories for a little while.

[lazzer408]

Originally Posted by favguy

This 600 watt unit looks like it might be a bargain, if one of you electronic guru types can tweak up the output voltage a little

http://cgi.ebay.com/New-PowerStar-60...QQcmdZViewItem

The ATX type power supply is similar but the low voltage cut off and sensitivity to over current spikes makes it less then idea for an EV. It's just to picky. Trust me I've already experimented on 3 different ones. I'm not saying it can't be done. It's just alot more work to mod it. You'd have to adjust the low voltage cut-off, adjust the current sensitivity, adjust or eliminate the over voltage protection so your volt mod doesn't cause it to shut down...bla bla bla. pita.