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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?

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  • \$\begingroup\$ how do you expect to charge a battery to 4.2V if you are providing only 4.1V? \$\endgroup\$ – jsotola Jun 16 '18 at 0:18
  • \$\begingroup\$ I don't. I don't want to overcharge it. Anything beyond the 3.7v the battery is rated for should be enough to keep it decently charged. \$\endgroup\$ – Jake Jun 16 '18 at 0:26
  • \$\begingroup\$ Nobody is going to be able to say that it is safe to float the battery at 4.1V indefinitely. Intuitively, it seems like it should be OK. But I have never seen a charger that did that nor have I seen a cell datasheet that suggested it could be done. I suggest you use a standard charging IC that runs the battery up to 4.2V (or 4.15V) and then stops charging until it is depleted somewhat. \$\endgroup\$ – mkeith Jun 16 '18 at 6:59
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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.

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  • \$\begingroup\$ Using two lead acid cells is an interesting idea that I don't think I've seen suggested before. \$\endgroup\$ – mkeith Jun 20 '18 at 19:55
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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).

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  • \$\begingroup\$ Trickle charge is a bit different than a constant charge, I am incorrect in my terminology. Trickle charging of a battery implies it is being supplied high voltage and low current. That is not what I am planning to do. I am planning to provide the battery both low voltage, and low current. Once the voltage of the battery reaches the same voltage as the power source; no current will flow. The battery will never reach 100% capacity, and therefore, will have no recombination issues. \$\endgroup\$ – Jake Jun 16 '18 at 15:11
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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.

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  • \$\begingroup\$ The battery is lithium ion. 3.7v 2500ma. Once the voltage of the battery and the power source are the same, would there be any excess energy to deal with? I am under the impression there wouldn't be a transfer at all, just a floating charge. \$\endgroup\$ – Jake Jun 16 '18 at 1:43
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Yes, you can do that, but the charging circuit must have a current limiting circuit to prevent it from attempting to provide current to the load during the intervals when the load is connected. Alternatively, you can use a double-throw switch to disconnect the battery during those intervals.

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