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Re: (ET) 8v battery in the mix?

It probably won't hurt anything on the tractor. The voltage tolerances needed to accommodate everything from an over-charged 36-volt nominal pack to a discharged pack is a big enough range that another 1.75 to 2.6 volts won't matter.
However, if you are using a single 36-volt charger to charge the whole string, you may damage the remaining 5 batteries by reducing their available finishing voltage by a bit over 0.06 volts per cell. Given the voltage range covered by most 36-volt chargers, this is likely a low risk. In effect, you're taking 2 volts off the charger voltage for the 5 6-volt batteries.
I expect the 8-volt battery will have a lower amp-hour rating (but not necessarily). If so, you want to shallow your discharges a bit so you don't over-discharge the 8-volt battery while the 6-volters are doing fine. You can't trust the single 'fuel gauge' for this. The 8-volter is already going to have it reading high, and the 6-volters will happily hide the sag in the 8-volter if you are looking at a single volt-meter for the string. I think you want a separate meter on the 8-volter, and if it reads below 7 volts under load, time to park and charge.
I definitely would not recommend this if the other 5 batteries were newish. I'm not convinced it's a good idea even with old batteries. However, some days, you need a lash-up to get the job done, even knowing it may not be the best idea. My biggest fear here (coming from my reality) is that a 'good-enough for today' solution somehow remains in effect for a couple of years. 

Darryl

On 5/26/2017 2:47 PM, tb wrote:

No it wouldn't have the same capacity, but I probably/perhaps don't need
it to--just until I can get a 6v.  It would cost very little to try, but
I don't know if it could damage anything.

By the way, it's an E12M--not one of those 'high-tech' big machines.  :-)

Thanks.

Thon


On May 26, 2017, at 10:05 AM, David Tiefenbrunn wrote:


I would not do that.  The 8v can't have the same capacity as the 6v if
it's is the same size.   I free exchange for a used 6v with the other
5 at end of life isn't a bad idea.  Might get you another year.
 Definitely not worth getting 1 new one in with the old.  It will be
wrecked in short order.



Sent from my Verizon 4G LTE smartphone


-------- Original message --------
From: A-M <amcool gbta net <mailto:amcool gbta net>>
Date: 5/25/17 11:58 PM (GMT-05:00)
To: et <Elec-trak cosmos phy tufts edu
<mailto:Elec-trak cosmos phy tufts edu>>
Subject: (ET) 8v battery in the mix?

My pack is probably 7-10 years old, and still does all I need it
to--or did until I found one battery open circuit this spring.

As I understand it, putting one new battery in with the other 5 does
no favors to the new guy.

My local golf cart dealer will basically exchange a 'decent' old
trade-in battery --when he gets one--for my dead one.  He's already
been through the spring rush though, so no idea when that might be.

He does have some trade-in deep cycle 8v batteries though, which seem
to be the same physical size.  Any chance adding one to the mix might
not be too much, until a 6v shows up??

I do have a couple of ICE riders I can use, if this is a bad idea.

Thanks.

Thon






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--
Darryl McMahon
Freelance Project Manager (sustainable systems)