Earth X

Esteemed advisors,

With reference: "Electrical devices don't know how the electricity is created (lead-acid, LiFePo, etc.), they only see volts and watts (thus Amps) of electrons flowing through the wires. " 

I have been researching lithium "jump packs"  - Some of the manufactures/providers specify compatibility with "Lead Acid Batteries Only".

Clearly this is at odds with the above refence (which makes sense to me). 

Do you have an explanation ??

I don't know the answer, but I have a theory. There is a big difference between lead acid and lithium when it comes to how much charging current they will accept (lithium will accept much more).

So, if you have a small lithium jump pack and connect it to a large lead acid battery, by nature the lead acid battery is limited in how much charging current it will accept. On the other hand, if you connect it to a large lithium battery, the charging current could go much higher.

All batteries are rated as to how much they can discharge. A small lithium connected to a large discharged lithium could easily be overwhelmed, exceeding the safe discharge rate of the jump pack.

Just a theory.

Ken Ryan nailed it.  LiFePo batteries in particular can absorb a HUGE amount of current during the initial charge cycle.  For most Rotax engines, it will take all the alternator can give, but only for a fairly short amount of time, after which the charge rate drops significantly.  The cheaper "jump packs" don't have a sophisticated BMS to prevent them from dumping all the Amps the "dead" battery can take, and can self-destruct in that scenario.  I would NOT purchase one that stated it could not be used on Lithium ion batteries...  There is a difference, and the price delta brings significant functionality to the party.

I have had replies from a number of jump pack suppliers.

When I added the requirement (in addition to emergency start function) to power a small refueling pump, one advised changing from his initial 500 amp pack, to a 2000 amp pack - is this "supersizing" style marketing or is there a genuine change in function from supplying a start system (high power over short period) to a small 12V pump (low amps over 8-12 minutes)?

Another advised his 1000 amp initial recommendation, would have no problem powering the pump.

James N Parker wrote:

Ken Ryan nailed it.  LiFePo batteries in particular can absorb a HUGE amount of current during the initial charge cycle.  For most Rotax engines, it will take all the alternator can give, but only for a fairly short amount of time, after which the charge rate drops significantly.  The cheaper "jump packs" don't have a sophisticated BMS to prevent them from dumping all the Amps the "dead" battery can take, and can self-destruct in that scenario.  I would NOT purchase one that stated it could not be used on Lithium ion batteries...  There is a difference, and the price delta brings significant functionality to the party.

There is another reason too. Lithium batteries cannot be safely charged after they are too deeply discharged. If a cell drops below 2.5V (NOT total battery voltage but individual cell voltage, which is 4.2 fully charged, 3.7 nominal - slightly less for LiFePO4 aka LFP) then trying to charge it can cause it to explode. A smart BMS like the kind in Earth X will prevent this. If it ever sees a cell drop below 2.5V then it will refuse to accept charge ever again and your battery is "forever dead" and needs to be replaced. But a lithium battery without a BMS will try to accept charge and bad things can happen.

Also you DON'T want to do maintenance charges with lithium batteries like you do with lead acid! A lead acid battery needs to be topped-off when possible to prevent sulfation. A lead acid battery that is not fully charged has lead sulfate build up and this is normal but unless it's removed by fully charging the battery, will slowly crystallize and will no longer dissolve by charging, so that's why you do maintenance charges for it. Lithium batteries don't like that. They like to spend most of their time over 10% charge but under 90% charge. They do NOT like being topped-off because charging them to 100% reduces their useful life.