Charging batteries with a bench supply

[paulca]

There are literally dozens of articles on this, but they tend to go off on tangents about charging different cell types and lots of warnings about lithium charging.

My question is not about the battery, but the power supply itself.

I have charged batteries on both my bench PSUs several times and they don't seem to mind.  However what brought me to post this was on Friday night I connected up a large 26Ah SLA to my DPS5020 and it came on.  The suprising thing was the PSU wasn't plugged in.  It was powering itself from it's output!  I quickly disconnected the battery until I plugged in the PSU.

Of course with the load off, connecting 12V on the outputs might also have side effects.

I remembered that Dave's PSU design videos had a section on that where he (at request of a viewer) tested what happened if he connected power on the output terminals while it was off and... smoke was released.

So I ... should I make sure the PSU is on and the load is on with minimal amps before I connect the battery?

How much risk is there in a properly designed PSU versus a cheap RDTech one?

[amspire]

One the most important things to do is to add a series diode to the battery. A lead acid battery has a lot of energy, and if the mains fails while the battery is connected, then without the diode, it can blow up many power supplies.

I have had an HP supply that was badly burnt internally this way.

In your car you were lucky, but there may have been some internal components close to failing.

As far as connecting the battery, as long as the supply us current limiting, you can just connect the battery. The power supply will be fairly useless for NiMh and NiCd batteries. I would be careful with Lithium batteries - I would prefer to use one of the real battery charger devices

[David Hess]

It just depends on the design of the power supply.  Some include series protection in case the output is powered without the power supply being turned on.

[paulca]

One the most important things to do is to add a series diode to the battery. A lead acid battery has a lot of energy, and if the mains fails while the battery is connected, then without the diode, it can blow up many power supplies.

Good idea.  Slightly cumbersome to implement, but a good idea.  10A diodes are LARGE, and the voltage drop across them varying means you can't charge lithium to 100% with the diode in place.

As far as connecting the battery, as long as the supply us current limiting, you can just connect the battery. The power supply will be fairly useless for NiMh and NiCd batteries. I would be careful with Lithium batteries - I would prefer to use one of the real battery charger devices

I very occasionally use the bench PSU to charge LiPo as it can provide more current than my LiPo charger, if I want to fast charge a 5Ah pack at 2C the LiPo charger runs out of puff at about 6A.  Similarly for the SLA which can take 7A, the battery charger runs out of steam, the bench supply can push it. 

***Obviously this is always under supervision.  If I even go for a coffee or for a wee I wind the current way back.

[bd139]

I do this all the time. The biggest failure point is if the supply voltage drops less than the battery voltage. Some old power supplies (Farnell ones in particular) are quite nicely engineered by some insidious bastard. They will quite happily charge away and then you have a power failure which will blow the arse out of the pass transistor.

If you find that the charger is powering itself from the load then there's probably a big reverse protection diode across the regulator which is a good thing.

[paulca]

If you find that the charger is powering itself from the load then there's probably a big reverse protection diode across the regulator which is a good thing.

The sparks on inserting the banana clip should have raised the red flag to me that there was current flowing, but I thought, maybe I'm just charging the output capacitor.  Then the thing powered up and I pulled the + banana quickly.

The Tenma throws no sparks, it just registers the battery voltage.

So my take away is.

1.  Only charge batteries with the PSU if I really need the extra kick (or accuracy) from them.
2.  Power the power supply first.
3.  Turn the load on at a sensibly high voltage and sensibly low current.

Then connect the battery and adjust the current up.  Hopefully preventing any back feed.  Except of course if there is a power failure.  Then the PSUs are on their own.  The DPS5020 a new board is about £30, the Tenma would be a completely replacement at £100 ish.  Of course worst case is fire and death LOL

[bd139]

Don't rely on powering the power supply first. If the mains supply drops or the Tenma blows up (does happen) ... 

[mikerj]

One the most important things to do is to add a series diode to the battery. A lead acid battery has a lot of energy, and if the mains fails while the battery is connected, then without the diode, it can blow up many power supplies.

Maybe ok if the PSU has a remote sense that can compensate for the diode Vf, but otherwise it's far from ideal when charging the more voltage sensitive chemistries such as lithium ion.

I have a Keithley 2400 Source Meter which is ideal for charging single cells (or batteries with built in balancing).  It's a four quadrant supply so can be used for discharging as well.