Hi there, I have a project to build a battery charger that can charge 6V NiMH 9000mAh batteries. I have a reasonable understanding of what is required of the charger to do this, but lack the electronics knowledge and expertise to actually design one of these from scratch.
I would like to - here we go, the wish list - be able to plug the batteries in and have them fully charged in about 15 hours. Simple huh?
I've looked through various sites and come up with a few criteria for the charger I want to build.
The charging current needed will be about C/10, 900mA, and should be reasonably tightly controlled to achieve a full charge in a pre-set time period. That is the first thing I have not been able to figure out how to do. The battery voltage varies from; no voltage (when new) or more normally about 1V when flat, to approximately 1.45V fully charged. The charger voltage needs to be about 1.55V/cell, in this case 7.75V, in order to keep the required current flowing.
There are a few ways to terminate the charge after 15 hours, and this is where it gets a bit beyond me.
The best way, I've read, is to sense the temperature rise rate in the batteries dT/dt when they are fully charged to somehow turn the charging power off.
There should be a secondary way to turn the power off as well, quite often done by a timer of some sort. There are two schools of thought on these too. Electronic timers are good, but re-set themselves after a power out. Manually settable electrically driven mechanical ones are good, as they don't re-set after a power out and then deliver the charge in the required set time. They are also quite inexpensive. They can also be set for different charge times for different chemistries - NiCds for instance, as they don't need as long a time to charge fully. Just adds flexibility to the design I guess, for those that use both types.
As for plugs and connections, I have adopted the standard RCA type plug because they are cheap, robust, can handle the current, and don't pull apart too easily. They are also easy to get leads and extensions for.
Anyway, that is where the project is in my head at the moment. So, if you can shed any light on any part of it, or the whole thing, that would be great.
I would like to - here we go, the wish list - be able to plug the batteries in and have them fully charged in about 15 hours. Simple huh?
I've looked through various sites and come up with a few criteria for the charger I want to build.
The charging current needed will be about C/10, 900mA, and should be reasonably tightly controlled to achieve a full charge in a pre-set time period. That is the first thing I have not been able to figure out how to do. The battery voltage varies from; no voltage (when new) or more normally about 1V when flat, to approximately 1.45V fully charged. The charger voltage needs to be about 1.55V/cell, in this case 7.75V, in order to keep the required current flowing.
There are a few ways to terminate the charge after 15 hours, and this is where it gets a bit beyond me.
The best way, I've read, is to sense the temperature rise rate in the batteries dT/dt when they are fully charged to somehow turn the charging power off.
There should be a secondary way to turn the power off as well, quite often done by a timer of some sort. There are two schools of thought on these too. Electronic timers are good, but re-set themselves after a power out. Manually settable electrically driven mechanical ones are good, as they don't re-set after a power out and then deliver the charge in the required set time. They are also quite inexpensive. They can also be set for different charge times for different chemistries - NiCds for instance, as they don't need as long a time to charge fully. Just adds flexibility to the design I guess, for those that use both types.
As for plugs and connections, I have adopted the standard RCA type plug because they are cheap, robust, can handle the current, and don't pull apart too easily. They are also easy to get leads and extensions for.
Anyway, that is where the project is in my head at the moment. So, if you can shed any light on any part of it, or the whole thing, that would be great.
