[Mr Bags]

Can anyone enlighten me as to why the all electric eGolf has a heavy 'old style' lead acid battery under the bonnet? Surely the banks of lithium-ion batteries packed under the seats would power everything? Just mentioned this to a friend who has a Leaf, and he says it has a 'traditional' 12v lead acid battery as well. I'm genuinely baffled and confused!

[riverrock]

Apparently, there are a few reasons.
- The traction batteries are isolated when not in use (possibly for safety reasons), so there needs to be something to power always on things, like central locking.
- All the accessories (lights, etc) and safety systems (airbag, ABS etc) are the same for the ICE and EV versions (in the Golf anyway), so require a 12V electrical source. The Lithium battery system in an eGolf is 323V in comparison ( http://media.vw.com/doc/1142/2015_volks ... 87e9c3.pdf ). Other cars are similar - require a low voltage circuit for multiple things, with a 12V standard battery the easiest / cheapest way to achieve this.

[Mr Bags]

Thanks for that Riverrock - I'm still none the wiser, but then I'm not an electrical engineer. You'd think a 12v transformer would be cheaper and, as importantly, lighter - surely the 12v lead acid battery needs a constant charge, which must therefore come from the main battery? It would be ironic if my EV broke down due to a flat battery, despite the 'fuel' tank being fully charged!

I guess therefore that electric aircraft also need an additional 12v battery (with the additional weight issues) for powering radio/transponder/lights etc?

[MikeB]

The 12V battery on my I3 controls, amongst other things, the charging circuit for the main battery. Allows the main battery to be recharged from empty therefore. I presume that's the case for other EV's too.

Incidentally, BMW say you shouldn't try and separately charge the 12V battery - it is charged every time the main battery is charged apparently.