Can I provide a constant charge to a battery?

Question

I have a sketch here of what seems like a solid plan. sketch

I need to provide a .2ohm coil around 3.7v which will take around 20 amps. The load will be very brief; 3-5 seconds, and intermittent; 10 times per hour. Manually operated.

I have a battery that is rated for 25a max discharge and is known to be fine with the load. But I have to take it out and charge it...

I do not need the battery for mobility, but for the amps. The battery reaches 4.2 volts at maximum charge. I would like to provide it with 4.1v from a stepped down 5v 1amp power source to keep it topped off while the device is plugged in.

Will this work? Can I trickle charge a battery and use it at the same time?

Answer 1

Most 'top-up' chargers charge to 4.2v, then stop charging. Once the voltage has fallen back to 4.1v, they charge back to 4.2v again.

I cannot see that providing enough charge over time to a cell to keep it at 4.1v can be any more harmful to its cycle life than providing enough charge over time to cycle it between 4.1v and 4.2v. You appear to think the same way.

However, I've not found any reputable (ie from a manufacturer, from a university) source that says that leaving a lithium chemistry battery on permanent float charge at lower than 4.2v is OK. They only say that a permanent float at 4.2v is not, as the continuous current will eventually plate out metallic lithium. Manufacturers have put a lot of research into maximising the capacity, charge rate and discharge rates. A float charge at less than full capacity is financial anathema to them, so perhaps it's not surprising to have no data.

There are all sorts of fire hazards associated with using lithiums. As long as you have precautions in place (metal containment, no fuels lying about nearby), there's no reason you should not experiment with a lower voltage float if you want to. I would. Keep notes. Test their capacity every few months, and be prepared to change them. Maybe in a few years we'll have evidence for how well other charging regimes work. But you won't find any published source, or me, saying it will work as you hope.

If you want a solution that does have official backing, then 2 x 2v lead acid cells floated at 2.25v to 2.3v per cell will give you a nominal 4v.

Answer 2

If by "trickle charge" you mean "leaving the cell connected to a charge that in a form of another provides the cell with some power (e.g. a small constant current)" the answer is: ABSOLUTELY NOT!

You can't trickle charge a Lithium cell. NEVER!

It is extremely dangerous.

You can find all the details on battery charging in this application note from Texas Instruments: Battery Charging (SNVA557).

See also this page on Battery University site: BU-409: Charging Lithium-ion.

Relevant excerpt (emphasis mine):

Li-ion cannot absorb overcharge. When fully charged, the charge current must be cut off. A continuous trickle charge would cause plating of metallic lithium and compromise safety. To minimize stress, keep the lithium-ion battery at the peak cut-off as short as possible.

Concerning what you are trying to accomplish, since a cell is a two terminal device, you cannot physically charge it and use it as a source at the same time. The current either enters or goes out the positive terminal!

Of course you probably mean "can I leave the battery connected to the charger while the battery is under load?". Although conceptually possible, this is very risky for Lithium chemistries, because they are so fussy about how they must be charged.

To do that you would need a charger circuit that not only monitored the battery parameters (as every LiIon charger chip does) but it also compensated for any load variation. Therefore usually you end up with charger circuits that disconnect the battery from the load during the charge.

If the load is to be powered during charge (e.g. laptop PC), the power for the load is drawn from the charger, which acts also as power supply for the load.

Note that this is quite a different approach from some simpler circuits you see on the Internet which use NiCd or NiMH cells, which are inherently safer and less fussy (especially NiCd).

Answer 3

That really depends on the chemistry of the battery. Some chemistries allow (or are even designed to) working in a buffering mode - like a car battery - which is what you need in this case it seems. However, most lithium types do not permit trickle charging due to various reasons, mainly because lithium chemistry has no mechanism of converting excess energy to heat similar to recombination in lead acid batteries. Judging by your stated voltage, this is not a lead acid battery, so I'd start with determining the type of the battery and then review the idea based on that.