Questions about the voltages of a lithium jump starter I purchased
Summary: The jump start pack voltage of 16V + will drop to around the current battery voltage as soon as it is connected. There is good reason to expect it to be safe to use. *



The following is based on the characteristics of LiIon battery chemistry.
It is extremely likely that the equipment is 'fit for purpose'. As in all such cases, caveat emptor*

The charging device appears to be using 4 x LiIon cells. 

A typical Lithium Ion cell has a maximum fully charged voltage of 4.2V.
Anything in the range 4.0 - 4.2V can be considered charged. Lower Vmax leads to longer cycle lifetime in exchange for a moderate reduction in per cycle energy capacity.

A typical cell is quoted as having a nominal voltage of 3.6 ot 3.7 volts. This is the nominal average voltage across a discharge cycle.

So 14.8/4 = 3.7V / cell = expected nominal voltage.

16.3 / 4 = 4.075 V which is inside the 4.0-4.2V range.
This has the advantage of allowing a simpler and safer charge termination system if they choose to use it. It will not noticeably affect charged capacity.

Automotive electronics must be engineered to withstand "things that happen" in a typical automotive electrical system. Voltages over or well over the nominal 13.8V max of the system can happen under certain normal operating conditions.

The jump start pack voltage of 16V + will drop to around the current battery voltage as soon as it is connected. There is good reason to expect it to be safe to use*.



150 W Inverter 

The pack should be able to power a 150 Watt load (about 10 amps)- although it is possible that it will work well as a jump start pack while failing to power such a load.  If the inverter starts on the original 16+V the voltage will usually almost immediately drop to well under 16V under a substantial load. If it operates but does not provide much power, measuring voltages at the source and at the inverter terminals may help assesss why.

This is because most jump starting devices do not do function in the manner that they claim or that they are commionly believed to work. In almost all cases the devices do NOT power the vehicle starter motor directly. Instead, they transfer charge to the "dead" battery, providing it with the ability to return most of this energy in a short period when the next starting attempt is made. This is a fortunate but little appreciated 'feature' of typical lead acid automotive batteries. 

I once jump-started a vehicle using 2 x 6V 3 Ah batteries in series, connected by a length of bell wire. This was being used as a camping light supply - which job it did well. Maximum current available at cranking voltages at the end of the bell-wire would have been a few amps. 10 minutes or so trickle charging of the 'dead' battery allowed the vehicle to start. Over decades I have regularly jump-started vehicles using jumper leads and clip connections that would have been wholly incapable of providing cranking level currents directly. 



*Caveat emptor -> let the buyer beware -> It's your responsibility.



These diodes are potentially suitable. This is a Digikey selector guide of diodes of 10A plus, through hole or stud or plate mount. Unusually in this case you want high forward voltage drop. eg These cost $US0.64/1  in stock at Digikey and drop 1V at 10A. 1 or 2 in series would drop the voltage initially. A relay or switch across them will allow lower battery voltage operation without loss.  These have the advantage that they do not need heat sinking - but will get very hot at 10A - about 100C rise above ambient! (which they are rated for !).

These are 64 cents and drop 2V at 10A and need a heatsink.