i want to run a 12v motor scooter starter motor from a 24v battery. it draws abt 20A.
a resistive load is impractical [gr8 heat], as is center tapping the battery [bad for long term charge]. a pwm regulator or power buck/boost device perhaps?
yes, i'm electric starting my 800w genset on my gopher for increased range. i'll put a spool on the shaft and wind up the recoil starter rope on it. a brief pulse of power and.....

 

Finding a converter that will handle the load transients is going to be the biggest challenge. Let us know what you come up with.

 

what happens if it doesn't cope with them? does it never start, does it ramp?

 

what happens if it doesn't cope with them? does it never start, does it ramp?

If the load transient is severe, the voltage will drop until the control circuit can recover and bring both the current and the voltage up. If the current demand cannot be met, then the converter will drop to its knees and not bother to stop there. Unfortunately, you may have to test a few to find one that is suitable.

 

will this do?
https://www.ebay.com.au/itm/1238086...hcg5hP9J9/4iER0ojKY6ycQmE=|tkp:Bk9SR-iL9YjVYA

 

Link is not reachable form NA

 

ebay.com.au item num#

133726974164

 

Is your new Starter directly Gear-Connected to the Engine You are starting ?
Can You increase the Gear-Ratio between the Starter and the Engine ?
How will the Starter be mounted to the Engine ?
The amount of Current that the Motor will demand is DIRECTLY related to how heavy the
mechanical Load is that it will be turning.
Is the Motor going to have a heavier Load than it was intended for placed on it ??????
How much Torque does it take to spin that "800-Watt" Generator ?, ( how many CC's is the Engine ? ),
I'm guessing quite a bit more than a "Scooter-Engine", ( what ever that might be ).

If the Generator is close to the same displacement as the Scooter-Engine, it might work.
If the Generator-Engine is smaller than the Scooter-Engine,
just use the full ~24-Volts to run the Starter-Motor,
but keep in mind that if the Generator doesn't start "instantly",
or ever has a hard time starting,
You could burn-up that Starter-Motor very easily.
Also, the Gear-Ratios must be taken into account.
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generator 66cc 4st, scooter 150-250cc 4st. 91RON fuel, both chinese industrial ohv motors with carbs.. starter shaft abt 13mm, normal reduction by gears 5-6in dia. genset pulley abt 4 1/2in. will externally weld 2 large outward dished washers to shaft and drill rope feed out hole at inner dia of far washer. starter bolted vertically beside genset on common steel plate with top bracket. it just spools up the rope to start genset. recoil spring will wind it back. choke, primer and tap by central locking solenoids.

 

it draws abt 20A.

Is that the running current or the locked shaft (stalled) current? Any supply/adapter needs to be able to handle the latter, which is usually five to ten times the former.

 

In any case, a "rope" will not work.
You will need a Cogged-Belt, or Chain, or Gears,
with a strong "one-way" Clutch, or other stout means of disengagement from the Engine.

Welding anything to the Motor-Shaft runs a very high risk of ruining the whole Starter-Motor.
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Is it a brushed DC motor? Or something cleverer such as a so-called “brushless dc” motor?
if it is an ordinary brushed motor, just switch it with PWM @ 50% duty cycle using a big FET.

 

High power 24 to 12V converters are common and cheap because they are used by people with 24V trucks.

Example

 

High power 24 to 12V converters are common and cheap because they are used by people with 24V trucks.

Example

Maybe today, I haven't kept up. But back when working on semis for my FIL his semi tractors used two 12V batteries with a connection to one of them for the lights and radios. No converters.

 

I am only going by the number of them I can find for sale and the price. Whether or not they are actually used by truckers is irrelevant.

 

You'll never run a 12v starter motor off that 20A DC-DC converter, the starting current will put the converter into current limit, especially if it's pre-engaged.. You might get lucky with a 100A+ converter but I wouldn't bet on it.

A 12v motor will run on 24v for short periods - it will try to run at twice its 12v speed. A DC motor controller that will operate at 24v will run a 12v motor at its rated speed at the 60-65% level. This one might do the job. Its 70AUD from China.





Alternatively, @Ian0 has it right - roll your own motor controller & run it off 24v with a 150A-capable MOSFET switched 60% duty cycle, you'll get the required hi-current starting torque and a 14v average run voltage. You'll need something like a IXFN400N15X3 MOSFET in a SOT227 case with screw terminals to handle the start current and a simple 555 timer to create the 10 - 12v gate drive. The only downside is those MOSFETs are 70AUD each and you'll need a certain amount of expertise to build it.

 

my starter is just a electric motor pulling on the oem recoil starter cord. i'll use short, long-spaced welds with wet rag heatsink on the end cap and shaft. i will re-lube brg when cooled. afair i have a 12v 50A pwm controller that may handle 24v. it was for the cars' heater fan, but is spare atm. the starter motor is cheap, new, on hand and brushed. although it will run in bursts of 1-2 secs, i'm unwilling to quadruple its heat by using 24v. , it might be easy to arrange a ramped lower voltage pulse by modding the pot cct. that would take the shock off the ratchet pawl, which is plastic. honda make the best recoil starter ratchets. pity i can't get one to adapt in. the other solution was a 12v impact driver married into the ratchet start, but space is limited. i could center tap the batteries, but fear damaging their long term charge. pls tell me if i'm being silly abt that.
advantages of 12v impact driver, strong brgs, low shaft speed and hi start torque-disadvantage: heavy initial shock on ratchet mechanism.

 

It is generally considered bad practice to run anything of one battery in a series pair as either that one will end up undercharged or the other overcharged in time unless you recharge them individually

 

that's what i suspected. it's too fiddly to do individual charging, and tho' only a few AS is involved with each start, it would soon add up. i doubt i could get 2 independent 12v intelligent chargers so each had it's own on a common mains. time separated charging takes too long. also, a 24v charger and a small independent 12v one run together for the extra loaded batt sounds complex. i can't do the maths, but assuming 100AS charge difference, the smaller unit must not upset the charging of the batteries in any way. the 24v charger is 6 stage. would they have to be matched, or just a 1A trickle charger do?

 

A bit of a daft idea, but you could start it alternately off each battery. Needs two double pole relays, but that might be easy.