Lawnmower battery/charging expert needed! Or...

[Tx Bohemian]

Lawnmower battery/charging expert needed!
Or at least someone who has more knowledge than me on this (which probably leaves it wide open…)

I have a Craftsman 42” riding mower that has a B&S 21hp engine that I pull a 60” Swisher tow-behind mower that has a 14.5hp B&S engine.
(This setup is used probably once every two weeks for maybe a half-hour total)

Both mowers have their own batteries for starting. The tow-behind has a bad battery.

The last few times I used the tow-behind I needed to jump it off with the rider, so that got me to thinking- do I really need a battery on the tow-behind?
It’ll never be used without the rider pulling it.

 I was thinking to run 8-gauge wire (or whatever the battery wires are) along the towbar/hitch to the rider battery.

So here's my question:
Would that over charge the riders battery having both mowers charging back to one battery?

[Daniel Meyer]

I'd say it's not worth the risk...the battery on the tow behind represents a "load" on its charging system, and a bad connection or intermittent connection would likely result in pranging the charging regulator circuit. (removing the battery from a running engine stands about a 60/40 chance of blowing a modern regulator).

[Robert]

Probably would not have a problem but like Daniel said is it really worth it?

How about running the wire leaving the battery and using it as a jump then disconnect it or use a switch. That way its connected at the flip of the switch so you can start and the running has a battery or load on the switch keeps both systems separate. Plus you always have the wire installed it serves as a backup and you can have the option at the flick of a switch.

[¿spoom]

I'd advise against it first of all, because it puts all the charging load on tractor. The more you ask an alternator to put out-the hotter it gets. Second of all, that puts the two batteries in parallel, and to do that right you really should start out with two new, identical batteries since in effect they'll operate as one large 12v one. That's why things like golf carts use the batteries in series and end up as 36v or 48v systems. I'd just add a Battery Tender harness to the mower unit's new battery you'll be buying  and remember to put it on the charger 1x-2x a month minimum in hot weather.

[Big Rig]

Thoughts on a marine type double battery switch that would let you draw on one battery to start and charge the weaker battery while running...?????

[Pappy!]

First off chances are your tractor alternator does not put out enough for two batteries.
Secondly the voltage potential of each of the two batteries will be the big issue.
Your tow behind battery will have the biggest voltage potential yet the alternator and your parallel hook-up will trick the alternator into thinking you only have one partially charged battery....follow me so far?
The alternator will then try and charge until it sees the voltage potential it is looking for before limiting the charge.
The problem here is that the "good" battery will be overcharged while the dead battery is still being charged resulting in a shortened life for the good one.
To do this correctly you would need a two bank "smart" charger that will control both charging circuits individually.

[scooperhsd]

Maybe if you had some thing like they use for RV battery charging - one terminal for the vehicle battery, the other for the RV "Housing" battery.

[OnaWingandaPrayer]

Why bother with two batteries? Unplug the the alternator on the tow unit. Run your #8 wire back there to crank it . Once you have started the tow unit , the only added load should be the fuel cut off solenoid on the carb. The Rider charge circuit and battery should handle that .
Am I overlooking something?

[Jess Tolbirt]

Why bother with two batteries? Unplug the the alternator on the tow unit. Run your #8 wire back there to crank it . Once you have started the tow unit , the only added load should be the fuel cut off solenoid on the carb. The Rider charge circuit and battery should handle that .
Am I overlooking something?

this is exactly the way i was thinking, run your cables to the starter solenoid and take out the battery and the charging module on the pull behind..

[Tx Bohemian]

Why bother with two batteries? Unplug the the alternator on the tow unit. Run your #8 wire back there to crank it . Once you have started the tow unit , the only added load should be the fuel cut off solenoid on the carb. The Rider charge circuit and battery should handle that .
Am I overlooking something?

No, I think you get what I was attempting to explain. (and I think your suggestion is the most promising)

I really appreciate all the responses!
I guess my original post wasn’t clear enough.

I’m not trying to charge two batteries from one unit.

Actually, the opposite:
I was wondering if I eliminate one battery, will two charging units (mower motors) overcharge the one remaining battery? (running about ½ hour or so at full throttle)

Let me try again and maybe a bit clearer:
Two mowers, a rider and a tow-behind.
Each has its own motor and battery. The tow-behind has a bad battery and I use the rider to jump-start it.

My idea was to remove the battery from the tow-behind and run battery cables up the tongue/hitch and hook to the rider battery, which would be the same as I do now to jump the tow-behind with jumper cables.
The difference would be leaving the cables hooked up once started.

Hence the question of two charging units on one battery.

[Gryphon Rider]

The main problem to be considered when paralleling either batteries (I know this isn't being considered by Tx Bohemian) or generators/alternators is a mismatch of charging capacity and battery size.  If you add a small battery in parallel to an existing larger battery with an alternator sized for the larger battery, you could damage the smaller battery because it would be charged too fast by the relatively over-sized alternator, possibly causing excessive heat and therefore damage to the battery.  The other problem with two batteries is the possibility that they both need charging at the same, thus keeping the alternator running at its maximum output for a longer period of time, thus shortening the life of the alternator.

All that being said, I think that it would be safe to use the Craftsman's battery to start the Swisher mower with several precautions:
1.  Remove the bad battery from the Swisher.  A bad battery can overwork the alternator charging it.
2.  Keep the Craftsman's battery connected to the Swisher's electrical system while the Swisher is running.  The battery acts to dampen voltage spikes within the Swisher's electrical system.  Running any system designed for use with a battery without the battery has potential to harm electronics.
3.  DO NOT start the Swisher immediately after starting the Craftsman.  Wait until the Craftsman has had a chance to replace the charge taken by starting the Craftsman.  I don't know if this will take two minutes or ten minutes.  If the battery is discharged but able to start the Swisher, and the Swisher is started, The Craftsman's battery will take all the current it can get from both alternators.  This could cause it to be damaged by the overly-high charging current.  If you wait for the Craftsman to charge the battery before starting the Swisher, then the battery will take a lower current for less time to recharge after starting the Swisher.
4.  Keep the Swisher's engine in top shape so it starts immediately.  If the Swisher takes a while to start, it will more deeply discharge the battery, causing the same problems as in #4, above.
5.  Disconnecting the Swisher's alternator, as OnaWingandaPrayer suggested, would eliminate much of the concerns noted above.

All this being said, it is likely less trouble to simply replace the Swisher's battery and use everything as designed.  This is what I would do.

[OnaWingandaPrayer]

.............All this being said, it is likely less trouble to simply replace the Swisher's battery and use everything as designed.  This is what I would do.

True, but I think an unneeded cost to maintain two batteries.

[Challenger]

Why bother with two batteries? Unplug the the alternator on the tow unit. Run your #8 wire back there to crank it . Once you have started the tow unit , the only added load should be the fuel cut off solenoid on the carb. The Rider charge circuit and battery should handle that .
Am I overlooking something?

As a retired Ag-L&G tech I can tell you we have had several customers with set ups exactly like that and running many years with no adverse effect. We wired one with an old winch plug-in so he could disconnect it quickly when not needed.