Adding a 12 volt battery to an 6-8 volt Pack

[rbuggies48IN]

Curiosity-thinking or crazy? I have 6 new 8 volt batteries and looking to make it a 60 volt system. Could I run the 12 volt battery in this pack? If so, what problems/issues would I have?

I am trying to do this on the cheap so I don't have to purchase another charger or separately introduce this additional battery in when I want to run at 60 volts by using another F&R switch wired up to do this ( thinking scottybs push button/lever HD unit). I would just use my existing 12 volt car battery charger on the 12 volt (which I would isolate each time I charge the 8 volt bats. I could also run 7-8 volt and separately charge the one extra 8 volt with a float charger/maintainer, but don't want to spend the $ 60.00 for this and not sure how long the charger would have to be on, etc.????. I have checked with the motor manufacturer and controller supplier and they will handle the 60 volts and so will the solenoid. I am guessing the right way to this is use the F&R mentioned to isolate this bat for charging and use it when running at 60 volts, but could bypass entirely with this set-up when I am not looking for the 60 volt boost.

[branson]

look in the extreme DC section 4 controller boost

the reason that your existing controller will handle 60 volt is because the charger will charge the batteries that high on the finish charge

on another note the things that you are trying to do are not really possible on the cheap

[Volt_Ampere]

Your fully charged batteries will be above 60V.

[Volt_Ampere]

IMO it's just a bad idea!

[rbuggies48IN]

Yep, 62.6 volts at peak charge on 6-8 volts is my latest reading. Question to my motor & controller manufacturer was whether they could handle a 60 volt battery set-up. I believed the motor would have no problems as I have read they can handle as much as 40% more voltage than factory rating Controller is good for 850 amps peak, but more voltage means less amp draw? I wouldn't call this a bad idea, just an idea that might work for me, but certainly not suggesting it will work for anyone else. Just trying to get to 60 volts before my batteries die in about 5 more years.

[JohnnieB]

Originally Posted by rbuggies48IN

Yep, 62.6 volts at peak charge on 6-8 volts is my latest reading. Question to my motor & controller manufacturer was whether they could handle a 60 volt battery set-up. I believed the motor would have no problems as I have read they can handle as much as 40% more voltage than factory rating Controller is good for 850 amps peak, but more voltage means less amp draw? I wouldn't call this a bad idea, just an idea that might work for me, but certainly not suggesting it will work for anyone else. Just trying to get to 60 volts before my batteries die in about 5 more years.

More voltage actually means more amp draw, but when a 36V cart is increased to a 48V pack, the amps are less because the workload stays roughly the same.

When changing voltages, you need to work in Watts to better see what is happening. (Watts = Amps X Volts)

For example: It takes 10 ft/lb of torque at the motor's output shaft to climb a 15° incline on 22" tires with four people on-board and to generate 10 ft/lb of torque, it takes about 4,800 Watts of energy. With a 36V pack, about 133A is needed, while at 48V, only 100A is needed. That is where the more volts less amps theory comes from.

However, it gets a little tricky because the RPM at which the motor spins when it is generating 10 ft/lb of torque under the same workload, increases as voltage increases, so the cart would travel faster up the hill at 48V than it does at 36V.

The dangerous part for the motor is at low RPM when the amp draw of the motor is not limited by the dynamic impedance and it will draw all the amps that are available to it. It this situation, more voltage means more amps, which in turn means more torque is generated.

An 850A controller can pass over 40,800 Watts to a motor turning at low RPM at 48V and over 51,000 Watts at 60V. Since the amount of heat generated inside the motor windings is the square of the amps of all the amps not converted to torque, and the workload imposed on motor by the cart will be roughly the same at 48V or 60V, the motor may get hotter during a max effort acceleration from a stand-still at 60V that it does at 48V.

[yurtle]

Originally Posted by JohnnieB

More voltage actually means more amp draw, but when a 36V cart is increased to a 48V pack, the amps are less because the workload stays roughly the same.

When changing voltages, you need to work in Watts to better see what is happening. (Watts + Amps X Volts)

For example: It takes 10 ft/lb of torque at the motor's output shaft to climb a 15° incline on 22" tires with four people on-board and to generate 10 ft/lb of torque, it takes about 4,800 Watts of energy. With a 36V pack, about 133A is needed, while at 48V, only 100A is needed. That is where the more volts less amps theory comes from.

However, it gets a little tricky because the RPM at which the motor spins when it is generating 10 ft/lb of torque under the same workload, increases as voltage increases, so the cart would travel faster up the hill at 48V than it does at 36V.

The dangerous part for the motor is at low RPM when the amp draw of the motor is not limited by the dynamic impedance and it will draw all the amps that are available to it. It this situation, more voltage means more amps, which in turn means more torque is generated.

An 850A controller can pass over 40,800 Watts to a motor turning at low RPM at 48V and over 51,000 Watts at 60V. Since the amount of heat generated inside the motor windings is the square of the amps of all the amps not converted to torque, and the workload imposed on motor by the cart will be roughly the same at 48V or 60V, the motor may get hotter during a max effort acceleration from a stand-still at 60V that it does at 48V.

=, not +. I do that all the time. Same key.

[JohnnieB]

Originally Posted by yurtle

=, not +. I do that all the time. Same key.

Shift key got me (again).

I corrected it. Thanks for the catch.

When I try to type with more than two fingers and think at same time, bad stuff appears on the screen.

[rbuggies48IN]

JohnnieB

Thanks for the info - always much appreciated. Now I am interested in how the bat-pack will react to this configuration, i.e.; 6-8volts with one-12volt. I am assuming the 12 volt, having less lead, will be my weak link for run-time and I should monitor it separately so I don't drain-it-down too low before my separate recharge.

[JohnnieB]

Originally Posted by rbuggies48IN

JohnnieB

Thanks for the info - always much appreciated. Now I am interested in how the bat-pack will react to this configuration, i.e.; 6-8volts with one-12volt. I am assuming the 12 volt, having less lead, will be my weak link for run-time and I should monitor it separately so I don't drain-it-down too low before my separate recharge.

I'm answering the highlighted question, but I'm not endorsing the concept.

The 48V pack (Six T-875) is rated at 170AH and the highest AH 12V available is 155AH (Crown, Deka or US Battery - Trojan max is 150AH), so the 12V battery will discharge faster then the 48V pack. The voltage drop under load will also be greatest on the 12V battery.

Yes, the 12V battery should be monitored separately and should be used as the run-time yard stick. Actually, you can monitor the 12V battery instead of the 48V pack since what it is doing is the worst case scenario.

System performance, range, Etc., will be limited to the AH capacity of the 12V battery. IE: The fact that 24 of the 2V cells have a 170 AH capacity is more or less meaningless since they are in series with six 2V cells that only have 150AH.

-----------
What are your plans for a battery pack when the 6 x 8V pack dies?
Lithium?
5 x 12V? (60V)
7 x 8V? (56V)
8 x 8V (64V)
6 x 12V (72V)