This is somewhat wrong. Yes a running vehicle can overload the system causing an mpem failure but jumping from a standing battery does not increase the voltage. 12v plus 12v in parallel like when you jump start a car ski etc. is still 12 volts. All your doing is doubling the capacity, Not the voltage. Now, If you went 12v positive on one battery to 12v negative on another battery and had a lead on the other postive and negative of both batteries hooked to say an electric motor, Now you have 24 volts going to the motor. Hope this makes sense.its the extra voltage that kills your mpem, a tender only puts out 2-4 volts, your voltage reglator( thats what keeps your batt charged while the engine is running) puts battery voltage up to 13-14 volts. the problem when jumping them comes in when you take the 10-11 volts from your "dead" battery ( yes even tho your battery is "dead" it most likely still has power on it, unless you have a drain on it and it took battery voltage down to 0 volts) and add to it the 13-14.5 volts from a running car or even 12.5 from a fully charged battery and you have 22-24.5 volts, which is to much voltage for a dieod on the mpem cuirct board to handle. it over powers it, causeing it to blow-out and making you either replace the dieod, or get a new mpem.
Lastly....the trickle chargers from Harbor freight are JUNK!!! Do not use them!!!
I have read many threads on them overcharging/wrecking the battery...With batteries almost hitting $100 and being as crappy as they are, it isnt worth skimping on.
Optimate 4...best charger out there.