Rechargable Batteries, Help!

[pteron]

These guys are battery suppliers to the model rc world and really know their stuff.

http://www.overlander.co.uk/batteries-chargers.html

Wow, why would you pay £3.05 each (£15.90 including delivery) from them when you could pay £6.90 for 4 from Amazon? http://www.amazon.co.uk/Sanyo-Enelo...7?s=electronics&ie=UTF8&qid=1337956730&sr=1-7

They may know their stuff but their prices are way out of line!

 

[CreakyDecks]

I cannot find a 12v operated charger for "D" and "C" cells. There are plenty of 240v operated chargers but I really need a 12v one. Any suggestions as to where I could look?

You don't need a charger. All you need to do is put a resistor between your 12V supply and the battery to limit the current flow. Use Ohm's law (R = V/I) to calculate the resistance you need for a desired current. Say you had two D types in a holder, the voltage drop would be 12-(2x1.2) = 10V (near enough) so if you wanted a 100mA (0.1A) charging current R = 10/0.1 = 100ohms.
The resistor would need to be able to dissipate one watt. Be careful because it will get hot. Alternatively you could probably use a suitable light bulb to limit the current, real high tech!
The more batteries you put in series (within reason) the less power you will waste heating up a resistor.

 

[noelex]

These guys are battery suppliers to the model rc world and really know their stuff.

http://www.overlander.co.uk/batteries-chargers.html

Get some Nimh cells and a 12 volt charger plus some plugs to fit the charger..

Get some battery holders, Maplins do them if Overlander can't help.

http://www.maplin.co.uk/search?criteria=C+Size+Battery+Holders

I use 8 cell holders and wire it up so that one side charges two at a time, the other four. N.B. you can not charge 4 + 2 at the same time the way I wire it.

Works for me.

One of the 12v hobby chargers will charge all 8 at once ( they need to be the same level of discharge). You can also charge drill battery's, lead acid batteries, in fact just about any rechargable battery all from 12v. It will give you statistics on how the battery is performing.
You do need to supply the battery holder as they only come with leads.
Many batteries can be charged quite rapidly, without working about resisters, or overcharging the batteries.

 

[pteron]

You don't need a charger. All you need to do is put a resistor between your 12V supply and the battery to limit the current flow. Use Ohm's law (R = V/I) to calculate the resistance you need for a desired current. Say you had two D types in a holder, the voltage drop would be 12-(2x1.2) = 10V (near enough) so if you wanted a 100mA (0.1A) charging current R = 10/0.1 = 100ohms.
The resistor would need to be able to dissipate one watt. Be careful because it will get hot. Alternatively you could probably use a suitable light bulb to limit the current, real high tech!
The more batteries you put in series (within reason) the less power you will waste heating up a resistor.

No! Don't do this. As the current drops the voltage at the cell will rise to 12V, you will massively overcharge the cell almost certainly damaging it.

If you did this to a Lithium ion, you'd have an explosion on your hands.

 

[noelex]

You don't need a charger. All you need to do is put a resistor between your 12V supply and the battery to limit the current flow. Use Ohm's law (R = V/I) to calculate the resistance you need for a desired current. Say you had two D types in a holder, the voltage drop would be 12-(2x1.2) = 10V (near enough) so if you wanted a 100mA (0.1A) charging current R = 10/0.1 = 100ohms.
The resistor would need to be able to dissipate one watt. Be careful because it will get hot. Alternatively you could probably use a suitable light bulb to limit the current, real high tech!
The more batteries you put in series (within reason) the less power you will waste heating up a resistor.

This system used to work well with the old low capacity Ni Cad batteries. Modern higher capacity cells are more easily damaged by overcharging. If you terminate the charge at the right time it still works, but that is difficult to do. One of the simple 12v chargers is much better or one of the hobby chargers if you want to have some more versatility.

 

[CreakyDecks]

I suppose I should have said NiCad! I assumed (carelessly) that anyone doing what I suggested isn't likely to fork out for Lithium C and D types. What I am talking about are NiCads which for C types typically have a capacity of about 1600mAh. If you left one of them charging for two days using what I described, massively overcharging it, it would still only be up to about 1.5V. But why on earth would you? Obviously if you choose not to use an intelligent charger you have to supply your own intelligence.

 

[pteron]

Ok point taken, but for the 8 quid that a cheapo charger costs from 7dayshop, I wouldn't take the risk.

To get the best out of cells you need to take care of them. I use the http://www.maplin.co.uk/mh-c9000-wizardone-charger-analyser-217893 but most wouldn't want to pay that much. If carlsberg made chargers though....

 

[greenalien]

Those electronic projects are great fun. If It uses the batteries in parallel and you build another one consider using 7 AA batteries.

It wouldn't work in this instance - the junction between the 2 batteries is the 0V rail.

Project details at http://nwavguy.blogspot.co.uk/2011/08/o2-details.html - great reading for anyone who's into innovative, and well presented electronic designs. There's also further info on his blog regarding Lithium batteries, that makes for interesting reading...