# How many watts are enough?



## TheSGC (Nov 15, 2007)

How are your running your A/C? Do you have another motor running the compressor or are you running it off the main drive motor?

750 Watts should be sufficient assuming your heater uses the pack voltage and you have a car battery to help out the DC-DC.


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## sailfish11 (Sep 15, 2007)

I'm still designing the system and deciding how to set it up, so I haven't got a heater yet -- although I definitely want one.

Is it best to have a battery backup with a converter? What purpose does that serve?


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## Qer (May 7, 2008)

sailfish11 said:


> Is it best to have a battery backup with a converter? What purpose does that serve?


Handling the current spikes. Some things on the 12 Volt system can use an awful lot of Amperes for short periods. Dimension the DC/DC for the peaks is very inefficient, thus you need a battery to level it out.


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## order99 (Sep 8, 2008)

I've always been curious about that-can you have just a 12V battery (Lead Deep Cycle, Lithium pack, whatever) on a separate circuit without the DC/DC converter for the 12V components? If so, could you still include it on the same Charger array or would you need a second stand-alone?


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## 1clue (Jul 21, 2008)

order99 said:


> I've always been curious about that-can you have just a 12V battery (Lead Deep Cycle, Lithium pack, whatever) on a separate circuit without the DC/DC converter for the 12V components? If so, could you still include it on the same Charger array or would you need a second stand-alone?



Yes, they call it an accessory battery and it does take a separate charger.


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## sailfish11 (Sep 15, 2007)

For the accessory battery, can you use a fairly small (i.e. light) one? Maybe a very small car, or even motorcycle battery?


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## 1clue (Jul 21, 2008)

I've heard comments about people using motorcycle batteries as accessory batteries. The size battery you need will no doubt depend on how much you use things like radio, lights, etc.

I've also seen recent threads on this site about how much a pain accessory batteries are and how they want to/are switching to a DC/DC converter for that function.

Read my signature: I'm not an expert here, just a n00b in spite of my high post count. I just get the impression that most builders think it's "better" to use a DC/DC converter than a battery. I suppose that the final decision needs to be made by you, with your own priorities. I get the impression it's more expensive to use a DC/DC converter.


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## Qer (May 7, 2008)

It can be a good approach to follow the golden KISS-rule and just use the original car battery as an auxiliary battery just to get the car rolling. Then, when the motor, controller and pack charger all works like a charm can you start looking at the less necessary parts of the conversion, like auxiliary systems, compartment heating etc. If you try to address all aspects at once, the risk is that it becomes overwhelming and that you never get to finish the conversion because of mental dead-lock.


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## sailfish11 (Sep 15, 2007)

1clue said:


> I've also seen recent threads on this site about how much a pain accessory batteries are and how they want to/are switching to a DC/DC converter for that function.


Right, but we were talking about using BOTH an accessory battery and a DC/DC converter. In that case, is the battery just for back-up and surges? If that's all it's for, then would a smaller/lighter battery work?


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## gmijackso (Sep 30, 2008)

I'd like to add that it seems many are leaving out the possibility of using multiple smaller DC-DC converters. For instance I picked up several 40-65V input (as I'm using a low pack voltage) and outputs just under 150 W of 12v DC (as well as 5V and 3.3V) for $5.75 each. I figure the 5V rail will work nicely for ICs, and 150W of 12V is probably sufficient for each side of exterior lighting (1 headlight, 1 set of led turn signals, 1 brake light etc).

Sure it might be a slightly higher pain in the butt, but it offers redundancy (if 1 DC-DC converter blows up, I still have 1 headlight and 1 brake light, better than none), and I think it'll save me some cash in the long run.


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## 1clue (Jul 21, 2008)

sailfish11 said:


> Right, but we were talking about using BOTH an accessory battery and a DC/DC converter. In that case, is the battery just for back-up and surges? If that's all it's for, then would a smaller/lighter battery work?



As I remember it, we were talking about either a battery for accessories or a DC/DC for accessories.

You have a DC/DC converter for the traction motor if you're using a DC system, but that has nothing to do with the accessory supply.

There's no single right way to do this. You need to have your accessories powered, and if you want redundancy then you could use two DC/DC accessory converters or you could use a converter and a battery.

IMO, not all DC/DC converters are created equally. Most generally assume you don't care how much power they use, so they have a lot of waste. I personally think a single one that's reliable and efficient would be the best approach.

Keep in mind I haven't even started my car yet. I don't have the practical experience in an EV to give a strong opinion backed by experience.


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## sailfish11 (Sep 15, 2007)

1clue said:


> As I remember it, we were talking about either a battery for accessories or a DC/DC for accessories.


I looked back on the thread and you're right, someone had mentioned using one or the other. 

But the thread kind of drifted (how unusual, right?). I was originally trying to decide on how big a converter I needed. I could try to add up the power needs of all my components, but that would be tricky and I assume that many of them have peak needs.

I guess what I was hoping is that some people that have conversions would chime in and tell us their experiences. What size converters are you using and would you make any changes (bigger, smaller, battery back-up) if you did it again?


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## Jimdear2 (Oct 12, 2008)

sailfish11 said:


> But the thread kind of drifted (how unusual, right?). I was originally trying to decide on how big a converter I needed. I could try to add up the power needs of all my components, but that would be tricky and I assume that many of them have peak needs.
> 
> You mighrt keep in mind the the OEM designed in an alternator that was sufficient to run all of the normal loads in the vehicle plus extra for recharge for the starting loads.
> 
> You can't go wrong if you size your DC/DC converter to the original alternator output less any removed loads like A/C compressor and plus any installed loads like electric heating


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## sailfish11 (Sep 15, 2007)

Jimdear2 said:


> You can't go wrong if you size your DC/DC converter to the original alternator output less any removed loads like A/C compressor and plus any installed loads like electric heating


But isn't that really the trick? I mean we took out a lot of stuff and added a bunch of stuff -- all of that affects how many watts I need. You're right, that I could just add all that in and then add a buffer amount and I'll probably be alright. I was just hoping to get some pearls of wisdom from some out there that have done this. Something like "I went with 500 watts and I wish I'd gone with more."

Absent that, I guess I'll end up doing just what you said, but of course the cost of the converter goes up with wattage, so I'd rather not go too much higher than I need.


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## gmijackso (Sep 30, 2008)

This doesn't count as a pearl of wisdom since I haven't done my build yet, but I was planning on 4 of the 150 Watt DC-DC converters I mentioned above. My thoughts were, 150W for left side lighting, 150W right side, 150W radio/misc, 150W to power my motor controller circuitry and other DIY electronics. Each of the supplies should have some spare watts leftover and I can run interior lights off one, dash lights off another etc.

That's my plan and my $0.02.


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## sailfish11 (Sep 15, 2007)

I've been researching other components (e.g. charger and BMS) for a couple weeks and now my thoughts are back to the converter.

Part of my problem finding one is that my battery pack is going to be pretty high voltage (320V) and there aren't too many converters that can handle it. I've basically narrowed it down to three:
MES-DEA 400-1000, 1000W, 70A, $1600
Solectria DCDC750, 750W, 58A, $1600
CC Power C400, 400W, 32A, $650

So, for the same price, the MES controller seems better than the Solectria. But what about the CC Power? Is 400W enough? I'd much rather spend $650 than $1600.

Anyone have experience or advice?


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## Jimdear2 (Oct 12, 2008)

I have a suggestion 

Figure your maximum 12 volt loads and maximum duration e.g. normal load + heater/defroster + wipers + headlights and running lights say for a 1/2 hour going home (don't forget extra wattage for side trips like to the bank and such), then put in an auxillary battery sufficient sized to take you through that period with the assistance of your dc-dc converter. Next figure your normal loads and size the converter to run those and enough extra to recharge the auxillary batery. Converter output should be 13.5 to 14.5 volts to properly recharge the aux battery plus most accessories run better at that voltage.

I know this sounds somewhat circular but a couple of back and forthes with the available components and costs should give you an answer. The extra weight of the aux battery might just offset that $1000 difference in converters

Hope this helps,
Jim


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## Qer (May 7, 2008)

sailfish11 said:


> Part of my problem finding one is that my battery pack is going to be pretty high voltage (320V) and there aren't too many converters that can handle it. I've basically narrowed it down to three:
> MES-DEA 400-1000, 1000W, 70A, $1600
> Solectria DCDC750, 750W, 58A, $1600
> CC Power C400, 400W, 32A, $650


Or you could get two controllers that can handle 160 Volt each and connect them to the upper versus lower part of the main pack. This means the DC/DC has to be properly isolated between primary and secondary side, but you'd want that anyway.

I dunno. One little part of my brain is screaming "IT'S A BAD IDEA, IT'S GONNA BLOW UP!" but I can't figure out why it shouldn't work. On the contrary, it SHOULD work... I think...


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