# [moved] replace burn resistant

#### behrooz_chz

##### Newbie
Hi
I have a rechargeable vacuum cleaner and try to replace its batteries unfortunately due to connecting pols in reverse, after trying to charge the battery a resistance on the board has been damaged and burned, so charging circuit is very simple and I just need to find the ohm and watt of resistance, the power supply is 18v 150mA and set of batteries are 12*1.2(2200mA)=14.4v
on the board for charching the battery I have 2 element a Diode N4001 CD and a burned resistance, so if i need a 14v for charging batteris what resistance with ??? ohm and ***watt we need?

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#### betwixt

##### Super Moderator
Staff member
The answer really depends on the capabilities of the charger. If it is a proper current and voltage controlled charger, the resistor will be there to work like a fuse. If the charger is just an unregulated power source the resistor has to drop the difference between charger and battery voltages at the desired charging current.

If the supply is 18V and the batteries need 14.4V, it has to drop (18-14.4)=3.6V. What we don 't know is the charging current but if the charger is rated at 150mA and used at full load (likely!) the resistor value should be (3.6/0.15) = 24 Ohms and rated at (3.6 * 0.15) = 0.54W.

My guess is the resistor should be 22 Ohms and rated at 0.6W or more. I can't be certain without knowing whether the charging current is regulated and my guess is based on protecting the batteries from overcharge rather than setting any particular charging rate.

Brian.