Power Boost Switch

[Tezzadactyl]

Hi everyone, I'm thinking about creating a power boost switch for my old ez-go 36v Golf Cart.
The batteries are getting tired and will only take charge up to 80%.
The batteries last long enough for what I am doing, but the cart get ridiculously slow on a couple of steep climbs that I have to negotiate.
I'm considering connecting another battery into the system that is connected with a 400 amp isolator switch. I would introduce the extra voltage only on the steep climbs so it wouldn't be running extra voltage for any extended period of time.
The 36v pack consists of 6, 6v batteries. When fully charged they should have 38.2 volts.
When I reach these steep inclines, I really only want to introduce enough extra power to speed things up without burning out the speed controller. The motor I have been told can take up to 48v with no problems, just not sure how many volts the controller can handle without burning out, so don't to introduce too much power.
Does this sound like a feasible concept? What could be the pitfalls if any?

Cheers.

 

[audioguru]

If you increase battery voltage to the motor by adding a new battery in series then the tired old batteries must provide extra current and will fail before you get to the top of a hill. The speed controller burns out by supplying too much current, not more voltage and you do not know what is its maximum allowed current and you also do not know the maximum overloaded current the motor will use.

 

[gophert]

Hi everyone, I'm thinking about creating a power boost switch for my old ez-go 36v Golf Cart.
The batteries are getting tired and will only take charge up to 80%.
The batteries last long enough for what I am doing, but the cart get ridiculously slow on a couple of steep climbs that I have to negotiate.
I'm considering connecting another battery into the system that is connected with a 400 amp isolator switch. I would introduce the extra voltage only on the steep climbs so it wouldn't be running extra voltage for any extended period of time.
The 36v pack consists of 6, 6v batteries. When fully charged they should have 38.2 volts.
When I reach these steep inclines, I really only want to introduce enough extra power to speed things up without burning out the speed controller. The motor I have been told can take up to 48v with no problems, just not sure how many volts the controller can handle without burning out, so don't to introduce too much power.
Does this sound like a feasible concept? What could be the pitfalls if any?

Cheers.

Electrochemistry is a funny thing. The chemicals ALWAYS make the same voltage when used to make a cell. If that is true, how come a drained battery puts out lower voltage? Well, The batteries are putting out low voltage because there is high internal resistance inside the batteries. An accurate representation of a real battery is a resistor in series with an ideal battery. So the positive terminal of a "real" battery is actually a node between the battery's internal resistor and the resistance of the load - so you have a voltage divider.

The internal resistance of a battery increases as the battery discharges (I can tell you more about that later if needed). As that resistance increases, you can use ohms law to see that the apparent voltage at the positive terminal drops.

Now, your solution is trying to add an additional battery in series is reasonable (on the surface) but you must realize that your additional battery must pass through the internal resistance of your existing batteries. Pushing many amps through the internal resistance of each old battery will cause significant heating.

You can actually measure the internal resistance (check interwebs for technique and calculation), then you can measure the amps through your motor under boost conditions, then you can calculate how many watts are dissipated by each battery as heat from the internal resistance. At that point, it is your call.

You must realize that a lot of heat is generated quickly when you are talking about 10s of amps to 100 amps. Heating the electrolyte will cause rapid evaporation and completely killing the batteries and possibly damaging their containers.

 

[Tezzadactyl]

Thank-you both for your quick reply. As you can tell I don't really understand the fine details of how these things work. As you said, on the surface, it sounds like a good idea.
If I was to connect a 12v battery in parallel, what would that achieve?
My basic understanding of a bank of batteries, in laymans terms, is that any batteries connected in series will increase the voltage, but connected in parallel the voltage will not change, but the "punch" of the bank of batteries will increase.

 

[alec_t]

If I was to connect a 12v battery in parallel, what would that achieve?

The 36v pack would try to drive high charging current through the 12v battery and could damage it.

 

[JimB]

If I was to connect a 12v battery in parallel, what would that achieve?

Not much, but it may be entertaining to watch, from a safe distance.
(The devil made me say it!)

JimB

 

[dr pepper]

If you put a good battery in series with a bad one, or in parallel with part of the string you run the risk of forcing one of the bad batteries to reverse its polarity, which could well cause instant death of said battery.
I'd start by fixing a meter to the cart temporarily and see what the voltages get to around the course.

 

[audioguru]

If you put a good battery in series with a bad one, or in parallel with part of the string you run the risk of forcing one of the bad batteries to reverse its polarity, which could well cause instant death of said battery.

Good point! Halfway up the hill the thing stops and needs to be carried home.

 

[crutschow]

Best you just replace the tired batteries.
Sounds like they're near the end of their useful life.

 

[audioguru]

The golfer uses a cart instead of walking around. Then maybe he is tired and near the end of his useful life.
I am 73 and run in the park with my dog, go for long walks with my wife, I ride a bike and I am very active. I have many more years to go.

 

[gophert]

The golfer uses a cart instead of walking around. Then maybe he is tired and near the end of his useful life.
I am 73 and run in the park with my dog, go for long walks with my wife, I ride a bike and I am very active. I have many more years to go.

Just think how active you would be if the OP straps a fresh, fully charged 18-year-old on you.

 

[Tezzadactyl]

The golfer uses a cart instead of walking around. Then maybe he is tired and near the end of his useful life.
I am 73 and run in the park with my dog, go for long walks with my wife, I ride a bike and I am very active. I have many more years to go.

You may be right audioguru, I might be near the end of my useful life. I won't embarrass you by mentioning why I am unable to run in the park etc. Trust me, if I was able to walk the course..... I would.

 

[Tezzadactyl]

Thanks everyone for your replies, I will check the voltages on my next run and see what it is doing. I may just have to put up with it the way it is for now until I can save up the $1800 for new batteries.
I was just seeing if there was an easy fix to give it a boost until then, looks like it can't be done.

 

[Pommie]

You could conceivably use a battery designed for models as a boost battery but that will cost nearly as much as new batteries.

Is there a reason that are reduced in capacity? Did you leave them in a state of discharge? Do they need the electrolyte topping up?

Mike.

 

[Tezzadactyl]

I've only recently purchased the cart and have no idea what the previous owner did with them. Electrolyte levels are fine. I think the batteries are just getting pretty old.

 

[rjenkinsgb]

Depending on exactly _why_ the batteries have deteriorated, an electronic "desulfator" may work.

Two of the common causes of lead-acid battery degradation are reputed to be:
Long-term use and/or overload, where the plates degrade and shed material which ends up in the bottom of the cells. That is irreversible.

Sulphation of the plate surfaces - an almost-inert sulphur compound can form on the plate surfaces when the cells are idle, which reduces the capacity of the cells.
That can supposedly be reversed by a gadget that puts high voltage pulses across the cell and breaks down the sulphate by electrolysis.

Background info: http://en.wikipedia.org/wiki/Battery_regenerator

I've played with such a device on old UPS batteries; I seem to get around a 50% success rate with significantly improved capacity after it's been on for a week or so, or no effect at all. That's on 12V batteries that will not hold any significant charge to start with after being on float charge for long periods. They are also gel electrolyte types which are not ideal...


You can build your own or buy them as kits / ready built.
eg.
http://cdn.hackaday.io/files/257411101302944/batt desulfator.pdf
http://www.barkeraircraft.com/files/Pulse3_web_layout_.pdf

It's just about as cheap to get a ready made one as buy the parts...
**broken link removed**


If you try one, I'd connect to each 12V battery pair in turn, giving each equal time over a few weeks - or get/build three and leave them permanently connected.

I added ferrite cores to my charger cables with several turns through each, to isolate the high frequency pulses from the capacitive load of the charger.

 

[audioguru]

Just think how active you would be if the OP straps a fresh, fully charged 18-year-old on you.

WOW!!

 

[Pommie]

A little story about worn out batteries. When we moved to Australia we spent the first year in temporary accommodation. All our furniture and stuff was in storage. In our new home we had horses and installed an electric fence. I had some 6V lead acid batteries that I used to power it and, due to neglect, they needed charging every few days. After a few months I noticed that the batteries now lasted about a month. The electric fence unit put pulses onto the fence and I assume if not shorted to ground these pulses end up back in the supply similar to the claims of battery desulphators. Anecdotal but I do believe desulphators work.

Mike

 

[Pommie]

WOW!!

Careful Audio.

Mike.

 

[Tezzadactyl]

If you put a good battery in series with a bad one, or in parallel with part of the string you run the risk of forcing one of the bad batteries to reverse its polarity, which could well cause instant death of said battery.
I'd start by fixing a meter to the cart temporarily and see what the voltages get to around the course.

I took the cart out for a quick round this afternoon. Fully charged it was sitting on 37.4v, under full load going up the rise, it drops down to 34.0v.