Charging SLA Batteries with an Alternator

[ArcMan]

I'm working on my lawn mowing robot, which is (was) a standard pushmower with a 6.5 HP gasoline engine.  I will add 2 wheelchair motors at 24 VDC, 11A full load each for the drive wheels.  My original intent was to use two 40AH SLA batteries (in series for 24V) to give me 2+ hours of run time.  I'm rethinking this now.  I may want to go with smaller batteries and an alternator like the ubiquitous Delco 10SI 24V.  I've never done an alternator charging system, though.  Will it be OK to connect the alternator directly up to, say, 18 AH SLA batteries?  I'm concerned about providing too much charging current, as the 10SI is capable of about 60A.  Does anybody have experience with this?

[steferfootballdude13]

1st off I have little experience with alternators, but...

If you have already bought the alternator you could hook it up to a multimeter, and if all else fails buy a potentiometer.

I know that you already bought the lawnmower, but they have self driven lawnmowers that are propelled by the gas engine. If you were to get one of those and if you have a considerable amount of mechanical engineering skills you could potentially create a rear differential drive with a series of electronically controlled gears. However probably very difficult, and you already have your mower.

[Kirk]

 I googled Delco 10 SI 24V and found an ebay listing that says "A New Custom Built Delco 10 SI Type Alternator:
This is designed for Isolated Ground systems such as:
John Deere 24 Volt Systems.
This will also work in Positive or Negative ground systems.
24 Volt
40 Amp
One Wire Hookup
Standard 3/8 belt pulley

SO you could check out a Deere wiring diagram.
I think that this type of alternator has a built in regulator.  If I am right all you need to do is spin it!
Good Luck
Kirk

[slo]

A 10si directly driven by a lawnmower motor will just barely reach its operating RPM and consume about 2-8HP depending on field current and 10si model.  Add many pounds of motors, alternator, and batteries while trying to mow with the remaining couple horsepower and it will eventually stall the engine when trying to drive and mow or at the very least degrade mowing performance.
 Just to double check my opinion, the quoted Output specs of 24v 40A = 960 watts or 1.287HP.  In order to charge the batteries it actually outputs a bit more 28.8v = 1152W or 1.545HP add in the HP required to turn the alternator fan, bearing and belt friction, inductive heating, rectifier and regulator losses and the input horsepower  is a fair bit higher.  28.8v x 11A x 2 drive wheels = 633.6Watts or .85HP leaving a max of 518.4Watts or 18A charging current for the batteries.  Which actually looks not bad until you realize that you need to derate the alternator for lower RPMs. 
Figure in that you are now using expensive gasoline to charge your batteries instead of cheaper mains AC.  Alternators are noisy electrically.  EMI, RFI, voltage spikes, ripple, loose regulation can all play havoc with microcontrollers and radio control gear.

[ArcMan]

Very good points slo, and a good analysis.  I really don't want to reduce my 6.5HP engine to 4.5HP for cutting grass.  Sometimes my grass takes every bit of the 6.5HP in the spring.  I also didn't consider that it would be more expensive to charge the batteries with gasoline than with line power.  Of course with all the money I'm throwing into this project, I obviously don't care how much it costs   

I think I'll just stick with the 40AH batteries and line charging.  Thanks all.

[Asellith]

Technically you don't need "charge" the batteries. Just supplement the power needed for the motors to run and reduce the overall battery needs. The best option for the system would be a mechanical drive system and small batteries. The motor would drive the blades and the wheels with some electronically controlled gears you can still turn. Then the smaller batteries mean less weight to carry around. But if you already have the larger batteries and are like me more electrically inclined then mechanically inclined then go for it. Or do what I do and hang out with mechanical engineers

[ArcMan]

It would take a mechanical miracle to do what my dual differential drive wheelchair motors can do.  Based on throttle and steering signals from my RF remote, each rear wheel can turn forward or reverse with variable speed.  The front has swivel casters.  This gives the robot the ability to turn on a dime or even spin in place.  I have used this scheme on other robots with very good results.

[steferfootballdude13]

It would take a mechanical miracle to do what my dual differential drive wheelchair motors can do.

Not a miracle, just a clutch.

[ArcMan]

Actually, a clutch is a simple friction plate device that allows the drive wheels to "catch up" to the engine-side transmission speed.  It does not provide precise speed control nor does it allow reversing of the engine rotation.

[steferfootballdude13]

If you were to use a servo, to control the clutch, then precision could be obtained... somewhat. Another problem would be that you would have to run either at full speed or disengaged most of the time (not what you want to do) or the flywheel would wear out too quickly. However a clutch would allow you to disengage the motor in order to switch gears ie: forward, reverse (More Mechanical Engineering Skills required).

[ArcMan]

Even though I'm a big fan of Rube Goldberg, I think I'll stick with electric motors and variable speed drives 
Thanks for the exchange, though.  That's where new ideas come from.

[Asellith]

Thats what this site is all about. Some of us know electronics some of us know mechanics.

[steferfootballdude13]

Thats what this site is all about. Some of us know electronics some of us know mechanics.

I however am fortuante enough to have an insignificant ammount of both.

[Asellith]

Well as long as you want to learn new things then you'll be ok. The tutorials on this site are great and will help you out a lot.

[Kirk]

Hey consider buying a new hydraulic ZTR mower and some big servos to control the valves

OR

Look at the old Snapper riding mower tranny.  It is a spinning plate with a wheel that slides on an intersecting axis.  The location of the sliding wheel relative to the center of the plate determine the speed and direction.

KWC