Battery capacity

In 1997 Ada was fitted with a Lucas A127 55 amp alternator charging the domestic and starter batteries. A couple of years later when fitting the bow thruster I fitted a second alternator, also a Lucas A127 55 amp unit, to charge the bow thruster batteries. About the same time I bought a Sterling advanced alternator regulator but, although I modified the alternator to fit the -ve earth field wire, I did not fit it as there was no problem with the batteries keeping up with our use of 12v power.

With time we added more 12v equipment and began to have power capacity issues. When I got round to fitting the advanced alternator regulator it did not work. I have since realised that marine alternators have isolated returns so following the instructions and connecting the -ve of the advanced regulator to the alternator case does not make a working connection.

About this time we had a problem with the bearings of the domestic/starter alternator so I decided to replace it with a higher capacity alternator. I bought a Lucas A127 70 amp alternator and as the connections were different did some further research about marine alternators. (It was then that I realised my mistake when fitting the advanced regulator.) Fitting the new alternator was straight forward and marginally improved the battery capacity but there were still times when there was not enough battery power.

The original alternator was repaired and carried as a spare. Researching the replacement alternator I learnt that the A127 alternator case has 2 configurations left handed and right handed. As one fited alternator is LH and the other is RH I researched how to change from one to the other.

Latest idea is the realisation that it would be better using the 70 amp alternator to charge only the domestic batteries and to use the 55 amp alternator to charge the bow thruster and starter batteries. This invokes changing the handing of both alternators. Then moving the 55 amp alternator to the position of the original alternator and the 70 amp alternator to the auxcilary position. With the current cabling, the 55 amp alternator is charging domestic and includes a relay. The 70 amp alternator is charging the bow thruster.

All -ve connects are common so no changes to -ve cables. Disconnect the +ve cable from the domestic batteries to the relay. Connect the +ve cable of bow thruster batteries to the relay terminal formally connected to the domestic batteries. A new 10mm cable will be required. Connect the +ve terminal of the 70 amp alternator to the domestic batteries. A new 6mm cable will be required.

Finally, fit the advanced regulator to the 70 amp alternator. For about £10 I can purchase a replacement regulator with a -ve earth field wire already fitted saving modify the existing one.

The alternators are here at home. The major issue is, before taking off the 55 amp alternator I did not detect any output so may not be working. I've changed the handing and it's ready to refit and test. If it's not working I'll bring it home and replace the regulator and rectifier having already watched the online youtube training video! I'll change the handing of the 70 amp alternator when the 55 amp one is proven working as refitting the 70amp alternator to charge the domestic/starter batteries is the fall back position. When both are proven working I'll change the cabling. When that is done I'll change the 70 amp regulator, as this can be done without dismounting the alternator, and fit the advanced regulator.

Then the question will be, how long will it take to recharge the batteries to keep the deep freeze, fridge, satellite TV and inverter working!

Further thoughts now ... I will need to replace the relay with a split charge diode unit. This is to prevent the bow thruster drawing high amps, through 10mm connecting cables and the relay, from the starter battery if the bow thruster batteries are discharged. There is a fuses in the cable that should give protection but it's a bit crude.