spinningmagnets said:
Thanks. Many people already have a 12V (actually 14V) car battery charger, and those who don't can usually buy one the same day locally, also cheap price.
I suppose, but some of these things are made in a way that the voltage regulation itself is done by the battery it's connected to. Using it on a different chemistry may not work and could lead to overcharge / etc. Depends on the charger design, which the average person won't have a clue about, or be able to even determine if there *is* a problem happening.
Since you're also presumably talking about RC LiPo packs, which aren't known for their spectacular QC, adding in the possibility of a charger doing bad things to them is just asking for a disaster at some point.
*if* high-quality packs were used, *and* a high-quality car charger were used, then it'd probably be safe enough. But since it sounds like you're proposing a general-use DIY method, I'd advise against it just because you can't control what people will use for these things.
Also, some of these chargers are simply a transformer with a diode, and use the battery itself as a "capacitor" to do the filtering from AC to DC. I don't know how hard that would be on an RC LiPo pack, vs a flooded lead-acid type.
If they're buying an RC LiPo pack for the spotwelder, they could also just buy a compatible RC charger at the same time. I expect it wouldn't cost any more than the (much larger/heavier/unknown-safety) car charger, that they would presumably have to go out and buy separately from the RC LiPo packs.
Since its unregulated, would there be a benefit to adding a resistor between the car battery charger and this theoretical 4S LiPo pack? Would a small light bulb work as a current limiting resistor?
A resistor is only going to limit current in that it'll waste power across it while current is flowing, so yes, it can limit the current, but it won't protect against overvoltage (in the case of a completely unregulated charger with potential for that).
If it has to dissipate a lot of watts, you'd also need a large metal-cased resistor (and possibly a large heatsink to bolt it to, or some way to actively cool it with a fan, etc).
Adding more parts for little control seems like it'd just be simpler (and possibly safer, and smaller) to just use the chargers designed for this purpose.
The only LiPo packs that are spec'ed for a pocket spot-welder are already very high current, so...does the LiPo high C-rate mean it can absorb the car-battery charging amps without harm? The Pb car-battery charger I have now has a 2A/8A/12A setting capability...
High C-rate for discharge doesnt' necessarily equate to high charge rates. So if high quality stuff is used, that does safely allow for higher charge as well as discharge, it'd probably be ok. But if you can't control the quality of the stuff being used, you run a risk of fireworks, greater the longer and more often it's used this way.
Sorry if it sounds like I'm being a spoilsport; it's just that from various posts here on ES and elsewhere, I'd say that most people, even those going so far as to build their own battery, still know nothing about them or the possible problems that can happen, or how to prevent them, or even tell if they are happening. That makes a fire due to improper charging that much more likely--and I wouldn't want that on my conscience.
The risks are high enough even with a regular RC charger on RC packs, used correctly....
