flagman1776 wrote:While we are talking about fuses... I know my "big" scooter has a breaker between the 2 Group 27 SLA batteries. Why is it BETWEEN the batteries and not on the +24V power out lead? Do they get away with a smaller breaker?
If I were to do a lithium conversion, where should I fuse (breaker) the pack?
In theory opening a DC circuit anywhere will disrupt the current flow as the current goes in one side of the battery (pack) and out the other... But ONLY if the short circuit current path includes the fuse... This means that best practice is to put the fuse as close as possible to the supply connections so as to minimize the chance of a short that doesn't include the fuse....
Putting the breaker between the two batteries in our typical two-brick lead setups does a really good job since that is usually a pretty short lead, and is well protected... Plus it protects both batteries as BM said earlier...
In a car where you have a negative ground (so pretty much everything on the car is 'ground') you will most reliably open the circuit if the fuse is next to the plus terminal.... We generally tend to fuse the + side by convention as well, but it really doesn't matter as long as the circuit gets interrupted...
When you have two different supplies in parallel like Expresso is doing with his addon, it gets more complex, and protecting gets more complex...
If the only fuse is between the batteries on the chair, then there is little or NO protection, as the fuse is bypassed by the add-on.... If a short occurs in the wiring between the main batteries and the addon, you might blow the main fuse, but the add-on would still be shorted... If the short happens between the two packs and the rest of the chair, the add-on would not be protected, and might supply enough current to keep the main fuse from blowing... Either way expect lots of smoke.....
If there is a separate fuse on both the main batteries and the add-on, a short would need to blow BOTH fuse before the current stops - since the fuses are in parallel, each is only seeing part of the current, so it might well take longer to have either get hot enough to open, possibly dangerously so...
This means I'd be inclined to make both fuses smaller than I might choose if just running a single set of batteries.... Other than that, it is not really practical to do much about protecting against a short in the wiring between the packs. However it might be a good idea to add a THIRD fuse in the wiring between where the two packs connect and the rest of the chair electrics...
ex-Gooserider