Jim,

I doubt the voltage of the main battery would be sufficient to fully charge the emergency ignition battery. However, if the main battery is on charge (either from a charger or the alternator), and the MAIN bus voltage is close to 14 V then, yes, the emergency ignition battery will be charged.

Same caveat as with alternator though - if the ignition battery is significantly lower than full charge the unregulated charging current may be higher than recommended by the battery manufacturer. A good AGM charger will limit both voltage and current. Your main battery charger has no control over the current going to the ignition battery.

I wouldn't trust a 5 year old ignition battery without running discharge capacity tests. It may be easier to simply replace it as you should be able to buy one for under $30.