gel cell battery question

[rodmo1]

Hi all,

     I recieved a used quickie 300 power wheelchair the other day with the intentions of turning it first into a r/c lawnmower.  It came with 2 8G24NH gel cell batteries (14.4 volts).  The one charged and tested good, however the other only measures 8 volts and refuses a charge.  So now it seems I need to replace the batterie(s).  They are rated at 73 AH.  To do this as cheaply as possible I would like to replace them with 20 AH lead acid batts. for affordability and also in the event that if the motor controller is shot I can then use them on my motorcycle.  The original config. of the chair is rated for a 35 mile range and max speed of 7 mph ( I believe with no passenger weight).  I would like the r/c mower to run solid for minimum of 45 minutes.  Does anyone know if this is possible with the given capacities or would anyone have other suggestions regarding battery selections.  I should mention that the mower itself is gas powered.

Rod M.   

[Soeren]

Hi,

Too many variables to give a precise answer about run time, but if a 73Ah battery gives you 5 hours, a 20 Ah battery seems to be ample for 45 minutes.
But... You need a deep cycle battery for your lawnmover, while your MC needs a starter battery - two different animals!

Why do you need 2 batteries at all?
When you are done with your lawn, you should have ample time to recharge before it needs attention.

[rodmo1]

I need 2 batteries due to it being a 24 volt system with the batteries in series.  I guess what I really need to know is whether there is any significant difference between deep cycle gel cells and automotive (starter) batteries except for amp hours.  Basically with the system the way I would like it would have 25% the amp hours of the original configuration.  I think this would probably fit the requirements but I am unaware if there are any other aspects of the two batteries that would make it a no go.  The main problem is that the reccomended replacement battery is a couple hundred dollars and I am looking to do this project for minimum $

[Soeren]

Hi,

I guess what I really need to know is whether there is any significant difference between deep cycle gel cells and automotive (starter) batteries except for amp hours.

Oh yes.
Automotive batteries can deliver a large current for a short time, but dies quickly if you try to use them for deep cycling (i.e. charge, then discharge substantially, then recharge, etc.).
Deep cycle batteries have much thicker plates, so cannot deliver as huge currents (although they can still wreak havoc if shorted) as their automotive counterparts, but are much less affected by deep cycling.

They should not, repeat NOT, be used for the "opposite" purpose.

I have run a wheel chair motor at 12V in a 'bot (www.CupSucker.dk) and with me on top, it would still run faster than I considered safe after it ran me into a wall (controlled with a loosely wired potentiometer for steering).
How fast do you want your lawn mover to go?

[Admin]

I would like the r/c mower to run solid for minimum of 45 minutes.

How much power does your r/c mower need? You must either measure it experimentally or look at datasheets. If you know how much power you need, and how much battery power you have, you can easily calculate how long it will run.

[rodmo1]

Sorry for the overdue response.  Basically I want the mower to go ~5mph.  Any faster then that and it could be a little too dangerous.  In reading the responses it sounds like I have to go with the SLA batteries.  That is unfortunate but hopefully I can score some from work.  I have another question which may better be served by starting a new post but I will try it here.  Are there any motor controllers available on the market for dual motors capable of handling ~25 amps peak that can be directly interfaced with an R/C reciever?  I saw one item, cannot remember who made it, that did this but only handled 10 amps. 

[Admin]

Yeap, go to dimensionengineering.com for motor drivers.

Or if you want to spend a lot, but want even higher currents, look up the various sites that supply battlebot parts.

For all 'where do i buy this' type of questions:
http://www.societyofrobots.com/robot_parts_list.shtml