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I have an old 36v DC battery hydraulic indoor forklift.

The batteries are 23 years old, and no longer hold a charge for more than ten minutes, although they are a good counterweight. I have tried keeping them charged, but it just chews hundreds of dollars in power.

The forklift itself is quite useful as it's a compact reach truck with 4 metre high lift, in good complete condition. I only want to use it once a week in my small warehouse and front loading bay, about 30m long in total.

I priced new batteries, many thousands, or used batteries at least a thousand, and would still require frequent charging.

So I wondered about a DC rectifier supply, with a long mouse tail or catenary overhead wire feed.

The batteries are 2 units of 9 cells x 2v = 18v rated at 387amp hours, 5 hour rated.

Does that mean they can deliver approx 5kw continuous work per hour for a 5 hour shift?

I hooked up a medium size DC welder to the hoist, but it barely makes the hydraulic lift motor turn, and the drive motor won't start at all. It's rated at 135 amp 30% duty, or 74 amps continuous at 47vdc. 30% would be fine, use is very light.

I figured I could just look at the DC motor rating, but for the hydraulic motor I can't see any plate, and the drive motor is a wheel hub motor, and I can't see any plates on it. The DC feeds after the electro-mechanical relays are all about 1/2" diameter including jacket, which gives an idea of the draw.

My real question is how big a DC power supply or rectifier do I need to get this unit moving? Will some big old capacitors help by buffering the rectifier from the DC motor start current? Would capacitors with the medium welder work?

I was thinking a large old 3 phase DC welder, something that can deliver 200 to 300 amps ought to do the trick, but unfortunately isn't very compact. I was hoping to mount the welder to the roof of the forklift so the tail is AC, and doesn't suffer the low voltage power drop over a long cable.

If it's just not going to be mobile enough I suppose it would work as goods lift to a mezzanine floor.

The other option is give up and scrap it, wreck it or sell it, and get a manual or walk-behind unit.

TL;DR - Recycle or repurpose way of getting a few hundred amps DC @ 36v

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  • \$\begingroup\$ The batteries are 2 units of 9 cells x 2v = 18v rated at 387amp hours, 5 hour rated. Does that mean they can deliver approx 5kw continuous work per hour for a 5 hour shift? No - it means they can deliver 387/5 = 77Amps for 5 hours. Average power ~1.4kW. \$\endgroup\$ – Kevin White Aug 9 '15 at 3:25
  • \$\begingroup\$ If you want to go full MacGyver, 150-200A at 36V might be within range of a 3-phase synchronous motor-driven (3000 RPM) heavy-duty automotive alternator. The voltage regulator (if internal) would have to be modified. Maybe a 5-10HP AC motor. SMAW and GTAW welders are 'CC' constant (ish) current not 'CV'. \$\endgroup\$ – Spehro Pefhany Aug 9 '15 at 7:53
  • \$\begingroup\$ Ah,so the welder option will never work, thank you. Motor/generator is a possibility, I'll have a rummage. \$\endgroup\$ – David McNeill Aug 9 '15 at 8:28
  • \$\begingroup\$ I'm curious did you ever get this to work, I have a similar project to get a small battery forklift working who's batteries are all completely spoilt. Mobility requirement is not more than about 10mtrs from any AC power supply. \$\endgroup\$ – Russ Oct 18 '19 at 14:27
  • \$\begingroup\$ Nah, all got too hard and complicated and potentially dangerous. Scrapped the forklift and bought a Logitrans LF Mini 1002 - manual 1 tonne to 1.6m, which has been awesome, love it. \$\endgroup\$ – David McNeill Oct 20 '19 at 10:02
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Given the limited amount of run time that you need, I'd seriously look at using automobile (car) batteries. They will need to be decently-beefy batteries and should all be identical, purchased at the same time and hopefully from the same manufacturing lot.

Having identical batteries makes charging them easier. The real concern is that some cells are lower capacity than the rest and those cells will charge and discharge differently from the rest.

You will get decent run-time if you keep the batteries charging while you are using the unit. Instead of trying to build a huge power supply that has to run everything, use the batteries as an accumulator.

You will need 3- 12V batteries of about 100 AH capacity to get the 36 Vdc rating of the existing battery. If you have the budget, double the number of batteries (3 sets in series of 2- batteries in parallel).`

You will also need to reduce the amount of current that the existing charger supplies - it is sized for the existing battery and most likely can deliver enough current to damage the smaller batteries that you are going to use.

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  • \$\begingroup\$ Great idea. I have a big bank of 32 x 33ah 12v identical batteries available, didn't think to use them, I'll give that a try. \$\endgroup\$ – David McNeill Aug 9 '15 at 8:32
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Buy a cheap 2000A (or more) DC clamp meter (if you don't already have one), charge the batteries and take some measurements.

Try lifting the heaviest load you want to move, try moving it around your warehouse. see how much current (Amps) that takes. with this information you'll have parameters to specify a suitable power source.

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