# Mixing battery voltages in the pack



## ZeroGasoline (Jul 30, 2008)

If this has been discussed already, I apologize, I wasn't able to find it.

My situation:

I have a 72v system with a high voltage shutoff of 90v. Currently I have 6x 12v batteries in series for 72v. When I start to climb a hill, there is a significant voltage drop and a resulting speed drop.

My question:

I've read where others have added a 7th 12v battery to a 72v system to give it a little more oomph. My concern with this is the fact that I would be knocking right on the door of the over voltage cutoff when the car was fully charged. There would almost certainly be times where I would be unable to pull the car off the charger and drive off immediately (due to surface charge exceeding 90v). 

What problems would arise from adding a 6v or an 8v to my current pack? This would give me a little extra voltage for hill climbing but would keep me safely under the 90v cutoff. I understand the new battery would need to be of equal or greater capacity than the rest in my pack. Has anyone done this or know if it can be done?

Thanks


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## TheSGC (Nov 15, 2007)

I don't know if anyone has done it, but I have a few reservations about it-

First the battery would have to be the SAME Ah capacity as your current batteries, or the charger will not function properly, unless each battery has it's own charger. My other beef is that the actually battery specs will be different. The Internal Resistance will be different, the Puekert's Effect will be different, etc. One "lower" battery has been known to take down a whole pack, so have 6 "lower" batteries and 1 "higher" might do something similar after a few miles.

However, if all your batteries are indivually charged and your "extra" battery is at least equal to the Ah of your others, then I really don't see any harm in trying. Just base all your SOC off you 72 volt pack and don't include the "extra" battery since it will always be higher than the rest of the pack.

What speed drops are you seeing? 72 volts is bottom of the barrel, and even my 96 volt system will drop from 35 MPH to 25 MPH up some of these New England hills.


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## ZeroGasoline (Jul 30, 2008)

TheSGC said:


> What speed drops are you seeing? 72 volts is bottom of the barrel, and even my 96 volt system will drop from 35 MPH to 25 MPH up some of these New England hills.


There is only 1 major hill that I run into on my way to work each day. I'll approach the hill at 55mph and I'm doing 30 or 35mph by the time I crest the hill (depending on ambient temp - cold=30 warm=35). The speed limit in the area is 55mph.

I know 72 volts is definitely on the lower end of the EV spectrum but it was at the upper end of my wallet spectrum . 75% of the time I don't mind the reduction in speed - but every now and again I do get a few cars behind me that have to slow down also. I figured an extra battery would give me just a bit extra to keep me going over the hill at maybe 40 or 45mph.

After I submitted the post I asked the same question to a co-worker who is a physics major. We tossed it around a bit and came to a similar conclusion. Unless the 6v or 8v battery was manufactured in exactly the same manor that the others were you would probably run into some compatibility issues (such as IR, peukert's, charge/discharge profile, etc). If I could find one from the same company with the same exact specs (just lower voltage) I think it'd be worth a shot. That may prove to be a difficult task. 

I may end up getting a 7th 12v and try to plan my departure times better. 

Can anyone comment on adding a 7th 12v battery to a 72 volt string?

Also, if I ran my motor (Motor D&D ES-15A-6 48-72 VDC) at say 96volts, what kind of risks would I be taking? If the motor going to become less efficient because of the higher voltage? Am I going to melt thing? I know that it would require replacing the controller, but would the motor be able to take much more than it's rated voltage? My thought is, as your speed and current increases, the pack voltage decreases - if you ran the voltage at 84 volts average then the batteries under load would put out closer to 72volts. However, between the time you start out to the time you get up to a good speed and heavy current, you'd be running the motor closer to the 80 or 84 volts.


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## Zemmo (Sep 13, 2007)

I think adding the extra battery would cause more problems than what you are resolving. Your charge wouldn't be designed for 84 volts and the controller is probably only setup to use 72 volts. Your will always have voltage sage with every battery you use. Just some batteries are better than others and don't sag as much. Thats where the Internal Resistance information comes into play. When the Internal Resistance is low, the easier the battery can release its power and hence will sag less than a battery with a higher internal resistance.

What batteries are you using and how much is your voltage sagging (like 63 volts at 140 amps type example)? You could be loosing some power from your cable connections and or battery connections. Do you get hot terminals after a drive? Depending on which battery you have, you wouldn't want to sag your voltage down much past 63 volts. The general rule of thumb is to not sag them down past 10.5 volts per 12 volt battery or 5.25 volts per 6 volt battery.


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## Coley (Jul 26, 2007)

I have been running a 72 volt system and using batteries of all kinds to see what would happen.

What I have done is charge the pack (6-12 volts) with my main charger. At the same time charge No. 7 with a good charger.

Before hooking up No. 7, run the motor a bit, in neutral, to cut the surface charge.

Using a load tester, test the charge on No. 7, which will cut the surface charge there also.

You should be ok, (using all 12 volt batteries) when you add them together, after this.

My 20 amp charger used to put my (6) LA batteries up over 84 volts, but my Alltrax controller was never bothered by it.

I have added No. 7 a couple of times, but it didn't seem to make a lot of difference. I have experimented with other combos, but not nuch improvement.

I am now getting ready to go with the GEL batteries (full set) to see how they perform.

I was using 4 GELS and 2 deep cycle marine LAs. Ran fair, but needed different charging voltages.


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## Guest (May 1, 2009)

Why go to all that trouble. Upgrade your system to 96 or 120 volts. You will be good to go with that and you won't have to worry about over-voltage. You can sell your 72 volt controller and recoup some money and give someone else a good controller for a low voltage ride. 

Pete : )


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## ZeroGasoline (Jul 30, 2008)

gottdi said:


> Why go to all that trouble. Upgrade your system to 96 or 120 volts. You will be good to go with that and you won't have to worry about over-voltage. You can sell your 72 volt controller and recoup some money and give someone else a good controller for a low voltage ride.
> 
> Pete : )


Upgrading the system voltage is the _eventual_ plan. 

I'd go to the trouble of adding a 7th battery for reasons of pure economics. 1 new battery and cables would run about $150 or so, where as getting a new controller, motor, charger and multiple batteries would be more like a few grand.

I've done some pricing on a new system and now I'm working on getting the money released from my CFO (wife). I'd like to bring the system up to 120v - based on other conversions on EVAlbum I think I could get up to 70 or 75mph pretty easily. If I got a good head of steam like that my one killer hill probably wouldn't be a killer anymore.

Out of curiosity, what would you say fair price is for the following (about 1000 miles on all of them):

D&D ES-15A Series Motor - $800 new
Alltrax 7245 Controller 72 Volt 450 Amp - $775 new
Schumacher Battery Charger 12-72 Volt, 5 or 10 Amp - $300 new

I was thinking about 70-75% of the original price, but I'm not sure how well these components hold their value.


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## TheSGC (Nov 15, 2007)

EV Parts hold their value very well. Asking 75% of original price should get them sold quickly. 

What motor are you looking to get?


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## rmay635703 (Oct 23, 2008)

TheSGC said:


> I don't know if anyone has done it, but I have a few reservations about it-
> 
> First the battery would have to be the SAME Ah capacity as your current batteries, or the charger will not function properly, unless each battery has it's own charger. My other beef is that the actually battery specs will be different. The Internal Resistance will be different, the Puekert's Effect will be different, etc. One "lower" battery has been known to take down a whole pack, so have 6 "lower" batteries and 1 "higher" might do something similar after a few miles.


I think everyone is making this more complex than it needs to be, I would recommend the contactor bypass "turbo" option, Using this method you would have to buy a set of cheap ebay contactors, a cheap 6v/8v charger and some wiring. When you approach a hill that is going to be a problem let off the gas kick in the bypass to put the battery in series with your other batteries and go. The battery could then be any size, even poor condition low capacity and it would only be used when you actually need it. Once you no longer need the boost, let off the gas, kick it out good to go again.

Cheers
Ryan


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## Zemmo (Sep 13, 2007)

Here is the age old response...

Why not just have the extra battery connected all the time to help increase your voltage and lower your amperage, otherwise your just carrying around dead weight for the few times you use the extra battery.

Like I said thats the age old response and it could go either way.....


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## ZeroGasoline (Jul 30, 2008)

TheSGC said:


> What motor are you looking to get?


I'm on the fence about this a bit.

As far as the actual motor goes I'm leaning towards the ADC 8" 203-06-4001 8" 72-144VDC 17.5HP Single Shaft. I'd probably get this running at 120v rather than 144v due to budget and available space for batteries in the car. I think the 8" motor would perform much better than my 6.7" now and the ADC motors seem to be pretty reliable. This would always leave me room for growth in the future (bring it up to 144v). My biggest concern is the length of the motor as I don't have much room to spare in the engine bay.

With this particular motor, I believe I could buy it and run it at 72v for a while and upgrade my controller/charger/batteries as funds become available. Running this motor at 72v should give me marginal improvements over my existing motor - those may however be offset by the additional weight of the motor.


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## Coley (Jul 26, 2007)

Another angle is to just install No. 7 , No. 8 etc., in parallel with 2 of your pack batteries.

It won't raise the voltage, but will give more amps for climbing hills.
It also won't need a different charger, just a little longer charging time.

I have tried this and liked the results more than raising the voltage.

I have planned to install a 2nd 72 volt pack, in the rear of my car to increase the range.

I don't need speed as much as distance.


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## ZeroGasoline (Jul 30, 2008)

Coley said:


> Another angle is to just install No. 7 , No. 8 etc., in parallel with 2 of your pack batteries.


Would this not begin to create some imbalance in the batteries? If one or two of the batteries had twice the capacity of the others, wouldn't the remaining batteries consistently under/over discharge?


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## ZEVUtah (Apr 10, 2008)

ZeroGasoline said:


> Upgrading the system voltage is the _eventual_ plan.
> 
> Out of curiosity, what would you say fair price is for the following (about 1000 miles on all of them):
> 
> ...


Instead of replacing all these components, why not just change the batteries to something that can give you the power ( amps ) that you need. 

Instead of using those garbage 12 volt batteries, go buy some 6 volt golf cart batteries. The first time I built my VW I used 12 volt batteries and the performance was horrible. 

After driving the car about 6 weeks, I pulled out the 12 volt batteries and replaced the 144 volt pack with a pack of 6 volters and the performance was much better. The voltage sag on the hills was much less. 

If you keep the system at 72 volts you can just use the same motor, controller and charger. 

KJD
http://www.zevutah.com/


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## ZeroGasoline (Jul 30, 2008)

ZEVUtah said:


> Instead of replacing all these components, why not just change the batteries to something that can give you the power ( amps ) that you need.


This is on the table as well. The batteries I have now could certainly be qualified as garbage. 

I don't know if I can squeeze 12x 6v batteries in the car. I'm sure I could but I'd need to do some suspension work I'm sure. Would the voltage sag decrease some if I went with some quality 12v? Maybe some Trojan T-1275's? Or would I really need to go with 6v to see a difference?


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## TheSGC (Nov 15, 2007)

You could also go with 8 volt batteries. They are the same physical size as the 6 volt, but you would only need 9 of them. They are usually 170 Ah and have a decent track record. But the reality is 72 volts is the extreme low end of things, so I really don't know if changing batteries alone will do anything. However, if you upgrade to larger batteries, that means you have more weight that the little sucker has to move around.

The 8" ADC should be a huge improvement over your 6.7" motor. It's much more efficient and wound for higher torque.


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## ZEVUtah (Apr 10, 2008)

ZeroGasoline said:


> This is on the table as well. The batteries I have now could certainly be qualified as garbage.
> 
> I don't know if I can squeeze 12x 6v batteries in the car. I'm sure I could but I'd need to do some suspension work I'm sure. Would the voltage sag decrease some if I went with some quality 12v? Maybe some Trojan T-1275's? Or would I really need to go with 6v to see a difference?


My VW has 20 batteries in it, so I would think you can find room for 12 in your car. 8 under the hood, 4 under that back seat, and 8 in the cargo area. 
http://www.evalbum.com/2058

You may see a small difference with a T-1275 battery, but not a big change. I have a friend that used these in a Mazda and after 19 months he is already thinking about a new pack. 

In my car I used the Trojan T-125, but last week at Costco I looked at their golf cart battery and it was the the same AH rating and half the price of the Trojans. Might be worth checking into.

As for stronger springs, you might give these guys a call.
http://www.coilsprings.com/ 

Good Luck with your project,
Kyle


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## ZeroGasoline (Jul 30, 2008)

ZEVUtah said:


> In my car I used the Trojan T-125, but last week at Costco I looked at their golf cart battery and it was the the same AH rating and half the price of the Trojans. Might be worth checking into.


What's the price people are finding these batteries (or similar) for? The going rate seems to be about $170. If I could find something closer to $125 or so I may be able to make the jump.


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## ZEVUtah (Apr 10, 2008)

ZeroGasoline said:


> What's the price people are finding these batteries (or similar) for? The going rate seems to be about $170. If I could find something closer to $125 or so I may be able to make the jump.


http://www.backwoodssolar.com/catalog/batteries.htm

You can buy from these guys and then just pickup the product from the local Trojan dealer in your city.


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## ZeroGasoline (Jul 30, 2008)

ZEVUtah said:


> You can buy from these guys and then just pickup the product from the local Trojan dealer in your city.


That's a great price on those T105's - $135 each. The no shipping is even better if I can find a dealer somewhere around here (Central Maine).

Thanks!


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