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Old 05-09-17, 11:02
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Lithium / lead battery combinations

Hi - have fitted a pair of lithium batteries from Allen MacD to my car and was planning to use the old lead batteries as slaves to start the car first thing in the day. When fully charged, the lithiums are 13.38V but the lead batteries are 12.8V. Is it ok to use a lead acid battery as a slave to add capacity to a lithium battery install? People must be doing this without issue surely?
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Rob
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Old 05-09-17, 11:18
kano nordie kano nordie is offline
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If you use the lead/acid just to start and disconnect there should be no problem, left connected the higher voltage lithium's will discharge to the lead aid battery's
John
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Old 05-09-17, 16:30
asklepios asklepios is offline
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I wouldn't bother as when I did it (in ignorance), the Li pulled down to the lower voltage. I confess that the lead acid was old and in itself wasn't up to snuff. I would use another Li battery as slave ( if you really need this), as much lighter and easier to transport. A quick recharge from the Sat to Sun is probably all that is required, as the Li PO4 seem to charge quickly with the correct device
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Old 05-09-17, 18:12
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Hi Rob

As Kano says keep the lead/acid and the Li separate.

We've run all season with Allan's CCA540.

We start the car with a lead/acid 480CCA as the Lotus needs a good strong crank to get it going.

Once warmed up we unplug the lead/acid and plug in the onboard Li CCA540.

The Li has gone all season with no charging (other than the alternator) and is still fully charged.
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Old 05-09-17, 20:42
No Imagination No Imagination is offline
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As noted above, wire your lead acid directly to the solenoid plus terminal and a chassis earth via your Anderson plug. Wire the 'on boards' so they don't/can't ever be connected to the off board.

I do something similar and use a separate on board starter solenoid which can only connect the on board lithium.
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Old 05-09-17, 21:41
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Thanks all so far for comments. Word from Allen is that lead acid slave plugged in to start engine is fine so long as its removed after firing and not left connected.
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