Lead acid battery charging

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.

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??

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

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.

I have one from Canadian Tire that is even older. It still works perfectly despite an awful lot of abuse. It is rated at 16 A, but I have used it to try and start cars.

Crappy Tire gets a mention in this song - which is appropriate at this time of year: 

https://www.youtube.com/watch?v=8s24MAtRgQ4&ab_channel=LianaGuitarBabe

I recall a story that Canadian Tire sued a person for ownership of the domain name crappytire. They argued in court that is was an infringement on the company name. The judge dismissed the case tellling CT in order for the lawsuit to be valid they must believe they sell crap.