Lead acid battery charging

You can certainly do this - usually for cycle-based charging, a safe voltage cap would be 14.4V with a current limit that matches the manufacturer's limitation. You should manually terminate charge when the current falls to a low-enough level (varies depending on who you read) - excess charging can boil the electrolyte or cause significant bubbles to form in gel cells and loss of electrolyte causing permanent capacity reduction. For float based charging, 13.6-13.8V would be a better choice, some cells do not need current limiting under that condition and generally can sit there all the time without much degradation. The optimal voltage does vary slightly depending on ambient temperature.

The main pitfalls are that a reversed connection will either kill the power supply or kill the internal reverse-connected bypass diode which will then kill the power supply. Another pitfall is that many power supplies don't have a true "switch" of the outputs, so connecting a battery can result in a spark as it charges up the output capacitors. When the power supply is off, any bleeder-resistors or down-programmers may end up discharging the battery slowly as well.

But they're all pretty much no big issue. If you'd like, you could use a hefty external diode to keep the battery from draining into the supply but at the cost of heat and loss of voltage accuracy (unless using 4-wire connection). I personally just use a switch to isolate the battery and slam the switch shut after the supply is on and stabilised, opening the switch before turning it off.

- Gough

As Gough explains using a power supply works fine. One thing to think about is how/when to turn off when it is charged. Lead acid batteries will continue to draw current after they are charged.

Knowing what this residual current is can allow you to know when to turn off charging. It is different for different battery capacities.

As you suggest a bench supply works just fine as a current source. I consider it more of a battery tester than battery charger, it isn't something I would recommended you leave unattended.

I use a schottky diode and a fuse as preventative measure from ending up with a fireworks display :fire 

Another gotcha is if your OVP trips on your power supply it will crowbar the output terminals...

Crowbarring a lead acid battery is an unforgettable experience - I can testify to that. In my case it was an associate of mine who shorted a huge battery with a crescent wrench - it turned white hot and melted - fortunately it melted so much it fell off before anything exploded.

I'm all for do-it-yourself stuff but on this one, I'd make a trip to Canadian Tire and purchase a ready-made fix. While I am out I stop by A&W to get a burger my missus doesn't think I should be eating.

That's a good point - OVP should be turned off or set high enough not to trip. If it does, it can be bad ... depending on how the power supply is designed.

- Gough

I agree, on both points . Once it requires warnings on how to do it, if you didn’t know these already, go with the safe option.  It required 3 members to reach the ‘crowbar’ warning - where’s the 4th who remembers something else to be wary of??

It may have required three members to get to the crowbar warning ... but having an OVP trip on pure battery charging is very unlikely on most power supply designs because the OVP is "tracking" - i.e. it's set to the output setpoint of the power supply + some margin (usually around 2V). The only way a trip would happen is if the battery voltage was somehow 2V higher than what you set the power supply to - if you make a mistake setting this, then what happens depends on the power supply.

Some "crowbar" the load, but that means they apply maximum down-programming (i.e. they do what they can to try and shut down the output and drop the voltage). For single-quadrant power supplies of the cheap variety, this just means a slow bleed resistor - for some higher-end supplies, an active FET downprogrammer (e.g. sinking 40-200mA) is going to load the battery and creates heat until it trips overheat protection (assuming it has one and isn't properly heatsunk) or fails - but most I've tested do not and cannot provide a dead short (in the sense of a true crowbar) which would sink a heavy current. If you're afraid of this, your diode in series will probably open up, otherwise put a fuse in line.

Only in very high-end power supplies can you set the OVP either very tightly or below the output voltage of the PSU itself where it would trip as soon as the channel was turned on. As the current flow is outwards towards the battery and the battery should not ever be able to sustain a voltage as high as the charging voltage (settled voltage of a 12V lead acid cell doesn't exceed 13.2V even in the best of circumstances), OVP trip should not happen. The only way OVP usually will happen is if say some sort of active load (e.g. a motor generating back-EMF spikes under load) was also connected and producing spikes the battery couldn't absorb, causing the voltage to rise beyond the power supply voltage transiently. I've demonstrated this in the past for someone who didn't understand why hooking a 12V cordless drill to a bench power supply and using it was causing it to trip on OVP.

I've been charging car batteries, sealed lead acid batteries, lithium ion/polymer and even Ni-MH cells from bench supplies for over a decade. It's not that dangerous for lead acid as a chemistry - it's probably the most forgiving of all the chemistries.

After all, some of the "dumb" chargers one can buy consist only of a regular power transformer, a couple of LEDs, a few resistors and a bridge rectifier, leaving you to monitor the voltage yourself and boiling batteries in the process.

Not to discourage you from "buying" your way to a commercial solution ... but sometimes it's easier to do everything with what you already have. As for things to beware of ... hydrogen gas is one of them, as is acid fumes - do your charging in a ventilated space, as even "sealed" lead acid batteries have one-way vents that will release some gas especially when overcharged grossly or initially charged. Flooded cells require additional care - liquid acid can splash if mishandled.

Can you tell I've got a PV solar system in my room and five lead-acid cells that I use to charge my phones, torches and other USB gadgets from? *chuckles*

- Gough

Thanks :) Not saying it is likely that one may inadvertently OVP trigger. Just, on my E3648 it is a genuine crowbar lol

My lips have uttered the words 'Crappy Tire" as a substitute for "Canadian Tire" because of poor quality of products. My battery charger bought at CT some forty years ago may lack the digital display of todays unit or a handle (the plastic handle dried out and broke off to be replaced by two loops of 14 gauge wire) but still works as reliable as the day I bought it. The switch to change from 12V to 6v came in handy when I got the snow blower.