Starting engines with jump box & dead house batteries

[tpenfield]

Don’t tell anyone, but I had 7 AGM batteries die one winter when the solar charger was covered with snow.

I brought them back to life, one at a time, by connecting them to a good battery and a charger. It took about 1 day per battery, but they were fine.

 

[jimmbo]

AGMs are a bit finicky when it comes to accepting a charge when severely discharged. A Smart Charger with an AGM Option, can usually cope with the quirks, but to use a regular charger requires the extra battery for many hrs.

 

[boscoe99]

Is anyone suggesting that electrically powered devices be kept turned off to the maximum extent possible in order to help the alternator last longer?

Maybe run a live well pump only every so many minutes instead of continuously.

Come in before dusk so navigation lights don't need to be turned on.

Turn on the VHF radio only when it is needed to transmit.

Only run one MFD at a time.

 

[Lou C]

I don’t think so it’s just be careful with jumpstarts and keep in mind sobs boats only have a 55 amp alternator not like on the vehicles we have now a days with much higher amp output (130 is common).

 

[Mc Tool]

Let's clear up misconceptions. Non-permanent magnet alternators are used in engines because they charge more at low RPM, when compared to generators. However, they require some back voltage to energize (excite) the rotor, to produce any electricity.

They produce AC voltage with the frequency proportional to the RPM of the alternator and number of coils.

As such, if you had a completely dead battery and you tried to manually start an engine by rotating it over, an alternator would not charge the battery, and if the engine runs on battery power, it will likely not start. When you jump an engine containing an automotive-style alternator, the jump power backfeeds the alternator rotor, allowing it to charge the battery. You could certainly put some voltage on the battery, which would feed the alternator rotor, and then manual rotation has a better chance of it starting.

Alternators can have some residual magnetism, which would induce some voltage that could be fed into the rotor, and produce charging. This magnetism can dissipate over time.

Generators generally have a permanent magnet in them and do not need any back voltage to generate. They charge less at low RPM but will usually charge a completely dead battery.

Before sensitive electronics were included in engines electrical systems, jumping an engine was no threat to those electronics. I expect modern engines to have some electrical protection for their sensitive electronics, especially in the marine environment, but would still be careful when jumping them.

Just a rant. Sorry.

The back voltage you refer to is called excitation and its that which the regulator manipulates to control output voltage . Some alts are self exciting ( not usually automotive ) in that some of the output is directed to the excitation coil but you still gotta get them to spin in the 1st place .
I have a friend that was working at a small power station ( Monowai ) and I recall him explaining how the excitation there worked in conjunction with the wicket gates and vanes to regulate rotor speed and current output . That was an interesting little place to visit . Lake Monowai , lower south island great fishing . The power station is still used and is at the end of a long -ish canal and actually sits on the bank of the Waiau river . There is a small village there , originally built to support the power station but now mostly just peeps that want to live out of town live there.