farkus888

so I have 3 radios from RTR vehicles that I drive on a very regular basis [2+ times a week] and I have decided that its time to convert the radios to NIMH rechargeable to save on batteries. the radios I have are 2 associated xp-2 3-channel radios and one traxxas TQ 2-channel radio. the TQ radio doesn't even have a charging port but the xp-2 radios do. what I want to do is modify the radios so that there is a deans plug dangling out of the side where the charge plugs are and use the RTR chargers from my rc18t's and my own NIMH AA cells. for reference those chargers output 120mA @7.2V.I know enough about basic electronics to be able to replace the connector that is already on the xp-2 radios and to attach my own lines to the tray of the TQ radio and add my own plug to run the current to the batteries correctly. my question is aside from superficial and warranty related issues, can anyone see a reason why this is a bad idea? mostly I am concerned about the charge rates. I will be using 2400mAh or higher NIMH AA batteries but I have not decided on the brand yet. any input is welcome thanks in advance for your help.

guver

My Feedback: (325)

I think it's a great idea and will save you some bucks on batts. How about using the existing chargeing jack on the tx? and finding the appropriate mate. One thing that may be a bit slow is the rate of 120 ma at 7.2 volts tryiong to charge 9.6 volt packs to 2400 mah. If anything the voltage will be ok , and the rate will be somewhat reduced and just take longer.

farkus888

the only issue I have with using the original connector is that its a hassle to find them and probably involves a trip to radio shack, a trip that I loathe. deans are readily available at my lhs, I might even have all I'd need in my toolkit now. as far as it being a slow charge rate, I figure its a trickle charge. the plan would be to plug them in when I get home from running and leave them plugged in till the next time I want to run. if the only issue is charge time I may just get right on it, now I just need to pick some AA cells and acquire another rc18t RTR charger, shouldn't be hard to find someone giving one away. unless they have read this thread

richrd

Interesting idea Iam lucky at the moment all are Futaba plugs. Only down side I can see is the plug dangling an then get annoying “ in the way” and second the deans need a cover or anytime the plug touches metal say in the trunk bad things will happen. The best IMO Sanyo HR3U- AAA 2100, 2500 or new 2700 mAH. I am using the 2100 and 2500mAH an zero problems also they delta peak 0.8C fast charge great. Typ 10 minutes charge in morning off I go. PS I bypassed in internal diode protection on all my transmitters.
Rich

guver

My Feedback: (325)

I've been using all AA nimh with futaba plugs as well. I use the stock chargeing jacks. Speaking of jacks, I hope you don't mind me jacking your thread, farkus. It may help you anyway to decide.

It seems that my transmitters now don't quite have the range they used to. When I went to nimh I looked high and dry for a 9 cell holder and never found one. I later learned that my tx beeps at 8.5 volts and seemed to work ok so I forgot about it.

I'm now re-considering using 9 nimh batts and wonder if my range will be increased?
ps I'll still need a 9 cell holder too. I want to stay with AA.

farkus888

I kinda thought that switching to NIMH might lower my range a bit because of the lower voltage of NIMH as apposed to alkaline but I figured it would only be an issue for the first 10%-25% of the battery life because of the difference in discharge curves of the two types of batteries. alkalines have a steady drop in voltage over their entire life where NIMH runs 90% or so of its life at one voltage then drops all at once. I have even seen some battery information websites say that over the life of a charge NIMH actually has a higher average voltage than alkaline. personally I don't race, just bash so range isn't a huge issue for me. how much range do you think you lost? was it ~10% or ~50%? the first one I'd ignore, the second one would make me rethink my plans.

guver

My Feedback: (325)

I can't really say for sure. I use 2 different tx primarily with 12 models and noticed it is different with each one. I've been using nimh for a couple of years and is now showing up more and more. I did try some alks and the problem all but went away. I may have multiple issues with multiple models so it's hard to pin it down.

I see what you're saying about the last part of the discharge, what is it on alks?
My nimh seem to be a nice gradual curve down to the 8.5 volts.

farkus888

the first picture I uploaded is the discharge of a D cell alkaline battery at 500mAh discharge rate, I stole the graph from page 4 of [link=http://data.energizer.com/PDFs/alkaline_appman.pdf#search=%22alkaline%20discharge %20curve%22]this[/link] document. the second one I loaded shows discharge curves of NIMH batteries. and is stolen from [link=http://www.nimhbattery.com/techfacts3.htm]here[/link]. to make the comparison a little easier the discharge rate on the alkaline chart converted to multiples of C is .06C, and its still a sharper discharge rate than the NIMH where the chart is showing .2C discharge rate. [both charts are voltage over time at Xdischarge rate.] as you can see the alkaline cells drop almost instantly to the 1.2v that the NIMH cells hold almost their entire charge.

you said you have been using NIMH for a couple of years, have you been using the same cells that entire time? if you have, the reason you have been seeing this problem get worse might be that the cells need conditioning or they may have reached the end of their lifespan. NIMH usually get about 500-1000 charge cycles before they start to die.

farkus888

I just realized that I never addressed this comment, as for shorting the deans plugs, I would use females hanging off the transmitters so there would be no shorting. putting the males on the chargers like I do with all of my chargers. about cell choices I am beggining to think I am going with maha cells, good reputation and a little cheaper than the GP in 2700 form. I figure with less than an inch of cord and plug outside the controller its not really gonna get in the way down there at the base as long as I make sure its no someplace where it will get under the base when I set it down.

farkus888

the cells have been ordered. went with [link=http://www.newegg.com/Product/Product.asp?Item=N82E16817355002]these[/link]. both the XP-2 transmitters have been modified, didn't have time to do the TQ radio before work today. haven't modified the AE chargers yet because I don't have any male deans plugs laying around, I'll have to pick them up when I go back to the LHS to get a motor thats getting a comm cut. I'll post pics of the finished radios when they are all done.

guver

My Feedback: (325)

Those should do a nice job, please do post back if the range is reduced. I'm kinda curious.