Step Down 72v Lithium to 12v

[Aktundra]

I have two 72v lithium batteries I’d like to reuse for powering a small Bambi. I see 72v to 12v step downs, priced from $250 to $12! Not particularly savvy in this area I usually just touch wires till things work, or there is a flash, and I begin again after my sight returns !!!

The batteries recharge with household current converted to a 12vDC 3-5(?) amp trickle charge

What should I look for in a step down ?
Any thing most efficient with recharging ?

[kittmaster]

As an electrical engineer, my advice is to avoid this solution. I've worked on multiple power conversion systems and power systems, the delta of 50 volts reduction based on some power step down device just wreaks of things that could go wrong. Since nothing is infallible for failure...if this type device shorts, or malfunctions in any way.... you are going to literally destroy anything that is attached to that 72V source in an instant.

While I appreciate the value of higher voltage and assume higher currents that the 12v/100AH flavors, the risk you would be taking to the devices attached would be mountains of money to replace...not repair....replace....as most of those devices will not survive that type of voltage presented.

Even with 15V protection clamps on the line....once the clamp burns...it will be game over.

IMO, rethink it, not worth it for the if and when things go south. Sell the batteries and get something that is suited for that application.

[OrangeCrush]

I too am a experienced EE and agree with everything kittmaster says and will add to it. 72VDC is higher than the safe limit of most agencies (UL, CSA, OSHA). Though not hideously dangerous, the right conditions could result in some personal safety concerns. It is also higher than the insulation rating (usually PVC) for typical automotive cable which could present some arcing and fire potential. Higher voltage rating cable is not a problem but it is just an example of the gotchas that require some thorough thinking. Bottom line is though it is feasible this project does not fall in the "touch wires till things work" category so I too suggest it is not worth the risk.