Model Boat Mayhem

Technical, Techniques, Hints, and Tips => The "Black Arts!" ( Electrics & Electronics ) => Topic started by: Alfie G on October 08, 2011, 11:44:41 pm

Title: Battery/ Headlight bulb Query.
Post by: Alfie G on October 08, 2011, 11:44:41 pm

I have a ni cad 7.2v 1800mah battery for my rc boat. I was told they have a memory and you have to drain them completely before recharging them.  Is this true and if it is, how do i wire that up?

Thank you 

Title: Re: Battery/ Headlight bulb Query.
Post by: Umi_Ryuzuki on October 09, 2011, 02:05:42 am

Nicads do build up a "memory" due to crystallization of the chemicals in the battery, however,
the batteries should not be discharged completely, or there is a risk of reversing a cell.
One battery in the pack might, literally, change polarity, if the pack is over discharged.

Several manufacturers make dischargers that will shut off once the battery pack is discharged to its minimum level.
One of the simplest used to be made by DEANS, the manufacturer of wire plugs.
It could be hooked up to most battery packs, and a series of lamps and would disconnect automatically once
the battery was discharged. You should do some searches for something similar.

 :-)

Title: Re: Battery/ Headlight bulb Query.
Post by: Alfie G on October 11, 2011, 12:16:43 am

Hi there,

Thanks for the advice.  I found a few online like you suggested, but will see what my local model shop can sell me one for, if they have one.

Thanks again for your help.

Alfie

Title: Re: Battery/ Headlight bulb Query.
Post by: Alfie G on October 11, 2011, 01:36:08 pm

I asked at my model shop and they could sell me a battery discharger, but only as part of a charger kit which costs about £50 to £60.

Do i really need this?  Some of the chaps at my local club in Glasgow hook up a headlight bulb to the battery and disconnect it when it goes dim.  Would that work?

thanks for your patience.

Alfie

Title: Re: Battery/ Headlight bulb Query.
Post by: unbuiltnautilus on October 11, 2011, 05:16:41 pm

That would work BUT.... I have tried that a few times and  I forget there is a bulb attached to the battery, as the most obvious indicator, the light, has gone out! I have seen a relay used in conjunction with a load i.e. bulb. The relay drops out when the voltage reaches a level where it can no longer energise the coil. Problem is, I cannot remember the circuit wiring for this!

Title: Re: Battery/ Headlight bulb Query.
Post by: Alfie G on October 18, 2011, 08:17:26 pm

Thanks for your help.

Title: Re: Battery/ Headlight bulb Query.
Post by: malcolmfrary on October 19, 2011, 10:15:22 am

I did read an article a while ago (can't remember where, but it was on a link from a forum discussion (probably rcgroups) on this subject) where the "official" method of long term storage for NiCads was to totally discharge them, and leave them shorted, going through a long reforming process before use.  I took that to mean that different internal chemistry occurs between actively discharging a cell and just letting it discharge on the shelf.  Notice "cells" rather than "batteries".  In a battery, under discharge, one of the cells might be discharged before its mates, all things not being equal, and would try to reverse charge from the current produced by its mates, which would in turn prejudice against its full recharging, the probable cause of memory.
A good automatic discherger/recharger is probably a good investment, since it "should" sense the points at which the different things happen, and move to the different stages of the job at the right time.

Title: Re: Battery
Post by: Alfie G on October 22, 2011, 12:44:46 am

Hi everyone,

My local model shop said that my nicad batteries were illegal!! I explained the batteries came supplied with a boat that was a gift and i didn't know they were illegal.  It was as if i had robbed a bank.  Can anyone explain why they are illegal as the chap didn't explain why.  What should i buy instead?

Thanks everyone for your help.

Alfie

Title: Re: Battery/ Headlight bulb Query.
Post by: ACTion on October 22, 2011, 09:38:04 am

Alfie
It's not illegal to use   NiCAD batteries, just to manufacture and sell them after a certain date (now passed - can't recall when). As regards your headlight discharger, simply connect the bulb across the battery leads and connect a digital multimeter in parallel with the bulb. Set this on a suitable voltage scale (e.g. 0-20v). Your 7.2v pack has six cells and conventional wisdom says to discharge the pack until each cell is 1.0v. With the bulb and meter connected, the voltage will slowly drop; you'll have to sit and watch it as it accelerates for some reason!  When it reaches 6.0v disconnect the battery. Done.
DM

Title: Re: Battery/ Headlight bulb Query.
Post by: unbuiltnautilus on October 22, 2011, 09:44:27 am

Your local model shop is correct and completely wrong. It is illegal to sell through nicad batteries from distributor, to wholesaler to customer once existing supplies have dried up,ONCE supplies in the supply chain have been exhausted. I would guess there are very few nicads left out there, except maybe the finest quality product of China!
As an aside, if I treated every customer through our doors as a criminal ( no matter how swarthy, sneaky and untrustworthy they appear- Mayhemers excepted of course! ) I would be out of lifeblood in no time at all...

Title: Re: Battery/ Headlight bulb Query.
Post by: pamian on October 26, 2011, 08:50:12 am

you have blood???

Title: Re: Battery/ Headlight bulb Query.
Post by: Alfie G on October 27, 2011, 11:11:32 pm

Thanks for the clarification guys

Alfie