Modern battery charging relays - worth it?

[N147JK]

I'm trying to better understand the advantages behind the modern battery charging relays such as the Blue Sea ML series over a solenoid-based isolator. Maybe our electrical system gurus can help with these questions.

I understand the new relays will automatically isolate or combine multiple battery banks as appropriate. But with an "old school" solenoid-based isolator, can’t we do the same thing, just less automatically?

With a solenoid-based system, if I turn the ignition switch to “ACC” I will combine the batteries. When the ignition switch is OFF, they are separated, simple as that. So, for example if I want to charge all batteries using the onboard shore power charger, I simply switch to “ACC”. The batteries are combined and are being charged, correct? (Yes I know I have to remember to turn that off.)

Do these relays somehow better manage the charging of separate batteries? For example, do they provide separate and better regulation of separate batteries automatically and simultaneously? I’m not seeing them doing anything like that.

Any time the starter is engaged, an isolator will close and all batteries will be combined. Or is this not the case? If so, this seems like the ideal arrangement, as providing vehicle starting power will always take priority over anything else. So it seems the old isolators are already doing exactly what is best.

I’d spend some money and replace my old isolator, but I’m struggling to see much value. What am I missing?

Any insights appreciated!

Note: Mine is a simple 2 battery, 12V system.

[Scalf77]

The obvious one is the one you mentioned. With the manual solenoid you will need to remember to disable as needed. Not, a huge issue, but a lot of people like the automation part. I always liked to monitor the start battery and provide a charge when I thought it needed it.

The bigger reason, really kind of depends on your battery bank size, alternator size, and the engine.

The auto relays monitor voltage to determine when to connect and disconnect the two banks. If you had a reasonable discharged battery, were running other high electrical loads it is possible when the two banks were combined that the alternator could not keep up with the load, so the voltage will drop. When this happens with a voltage monitoring solenoid it will disconnect if the voltage goes to low. If you just hard your solenoid connected to the "run" signal, it would remain connected. This especially is not that uncommon, when pulling up to a stop light and the engine goes to idle.

A V10 might be a little more resilient to low voltage, I expect a 6.0 would not be.

[boywonder]

A few more points...in no particular order........and I think Greg has a fairly comprehensive sticky in this sub-forum.....

One downside to solenoids (mostly when using solar) is that they burn power and make heat when the coil is energized. Not a huge deal when driving around. One use case is when you are parked and solar isn't putting out much juice because the sun is low in the sky/cloudy, etc......a chunk of that power is going to just keeping the solenoid engaged, making waste heat and increasing the entropy of the universe.

Don't confuse "isolators" with "separators". Isolators use big fat diodes, and all diodes have a forward voltage drop when current is flowing, usually around .7V so if your alternator is outputting 14V your house battery is seeing 13.3. This can result in undercharging. Isolators are sooo last millenium, these days it's separators or DC-DC converters.

Surepower separators use relay coils and get hot just like your solenoid but they come in two flavors the 1314 is front to back (alternator to house battery) charging only and the 1315 is bidirectional (goes both ways). This is handy is you have solar connected to your house battery and want that to also charge the starter battery when the van is parked. This happens automatically. If you have a uni-directional separator or solenoid with house battery connected solar you can leave the separator/solenoid "on" to charge both banks while the sun is shining, but that relay coil will be burning power all night/week long if you forget to disengage it (and you will forget).

The Blue Sea ACRs (Automatic charging relays) offer additional features and some are solid state (big fat Mosfets) instead of a coil and as already mentioned are automatic. They also allow easy connection if you need to jump start off of the house batteries.

All of the above are dumb, they are just high current switches, letting alternator power flow to your house battery. DC-DC converters are smart chargers usually applying a smart 3 stage charge profile to your house battery when the van is running. These are required when converting to lithium house batteries. Two downsides of DC-DC converters are limited current to the house batteries and they aren't bi-directional.

The obvious downside to a solenoid on a switch is that you forget to disengage it while camping and draw your batteries way down, now the van won't start and you can't use the house batteries to start it either since they are all connected together and are all dead.

[N147JK]

Thanks for the replies. I just spotted a Blue Sea 7622 for sale at a great price - I'll install it and then I'll be up to date.