Is a B&D Simple Start safe to use?

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Originally Posted by DMGRS

I'd much prefer the 'basic' version, a small lead acid battery in a similar housing with a set of crocodile clips.
Opening the bonnet and connecting one of those is hardly more effort, and you can start straight away. Plus you don't risk frying anything!

I would be cautious about this, if you mean drawing starting current with a small capacity slave battery in parallel with the main battery. The current will be significant, and drawn from the higher voltage source. If its capacity is relatively small - a few ampere hours, its voltage will drop and the internal heat can warp or collapse the plates. Also have a mind to the capacity of the cables between the slave battery and the car battery for the same reason.

A