Method 1: Charging and Balancing Cells Using a Regulated Power Supply Unit
There are a few options available for first charging and balancing the cells. Using a regulated bench top power supply unit (PSU) is the commonly promoted approach and also the least practical and accessible for a one-off job on board – which is often the context in place when building a DIY system on an ocean cruising yacht. This process is very slow, inefficient and requires a regulated power supply unit and mains power for several days.
In some cases these constraints don’t apply or this method can be combined with the second method to “finish off” the cells, so the process is explained below, but you should prefer the second method described.
Never use a common battery charger: its output is unregulated and, even if it is able to hold without overloading and tripping, it cannot limit the voltage as the cells charge up. The guaranteed outcome will be a totally destroyed set of cells at best, or a fire. Don’t imagine for a second that you will be able to “see it coming” and prevent it. The voltage seems to remain constant forever and then rapidly rises without any warning.
You need an adjustable, regulated power supply unit to follow this process.
Parallel charging and top-balancing cells using a regulated power supply unit (PSU).
Voltage regulation is essential to ensure the target voltage cannot be exceeded.
First of all, power the PSU
before connecting anything to it and
never interrupt the mains for as long as there are batteries connected to it. Some PSUs are not well protected against reverse current flow and not intended for use with large capacitive loads!
If possible at all, use a PSU that is explicitly suitable to charge a battery; in doubt, use great caution as a mishap can easily damage it. If smoke escapes from it, you will never get it back in.
- With the output disconnected, set the voltage regulation limit at 3.40-3.45V and preset the current limit (if any) to a value that won’t overload the PSU. Refer to the manual as required. In doubt, always start with a low current limit and never exceed 80% of the rated output.
- With all the cells wired in parallel, connect the PSU, bulk charge and absorb until no current flows any more. The voltage will stay around 3.3V for a very long time before starting to rise. Charging this way can take several days. This will near-fully charge the cells without stressing them unduly, but don’t hold them at that voltage indefinitely. Keep checking up on them at least a couple of times each day. Briefly disconnect the cells and recheck the voltage limit setting on the PSU: better safe than sorry. Avoid charging the cells individually, or in batches; the whole process would take just as long, but would also result in some fully charged cells lying around for several days.
- Once the voltage has reached the PSU output regulation limit and there is no apparent charging current any more, disconnect the cells from the PSU and increase the output voltage regulation limit to 3.60V.
- Then, while standing by only, reconnect the cells and allow the voltage to rise up to 3.60V and stabilise for a few minutes; this normally takes little time and additional current, provided the cells were fully absorbed at the lower voltage. Whether you target 3.60V, 3.65V or even 3.70V is of no consequence or interest if you are actively monitoring the process, because these values are often reached seconds apart only.
- Disconnect the PSU from the cells again and wait. The cells should hold above 3.50V for at least 30 minutes. If not, bring them up again and hold them for a little longer until they do. At 3.60V, you may need to insist a little more than if using 3.65 or 3.70 volts; that’s all.
As with all unattended charging of lithium batteries, some very careful thoughts must be given to the potential consequences of a failure somewhere
Using a regulated PSU, a failure – no matter how unlikely – of the unit cannot be entirely excluded and there is no other line of defence in place. Hopefully, it would just trip, but if it didn’t, it could lead to a battery fire. Someone could also come past and interfere with the equipment during charging with the same outcome.