How to top up a 12V aux battery

How do I keep a 12V auxilary battery topped up in a EV?
I have a few low current consumers and I like to run the ignition key via the BMS. (Avoiding the 12V bootstrap issue and giving the BMS full control over ALL power consumers)

Can I just run the DC-DC powered 12V bus via a diode into the battery? Won't that potentially cause very high charge currents and is 12V enough to charge a 12V battery?
Lead acid chargers do all sorts of 'intelligent' stuff and seem to use higher values.

I could also switch the always-on battery bus via a relay over to the dc-dc sourced one and taking the battery effectively offline while I charge it with special chargers?

What do you guys say?

I think most 'factory' EVs use a dc-dc converter that can directly charge a 12v lead-acid battery. This means there is some kind of current limiting to protect against a fully drained battery, and the output voltage is around 14v, not 12v. In most cases, you could just run a dc-dc converter at a slightly higher voltage. A blocking diode is probably needed, depending on your dc-dc converter. The voltage drop in the diode needs to be compensated for, so you may need the dc-dc converter to run at about 14.7v.

True, none of the components should be bothered by a few more volts. I guess low tech would work just fine too.

PS: Looks like http://www.analog.com/media/en/technical-documentation/data-sheets/40001fa.pdf kind of is what I want. In fact pretty much what linear calls PowerPath is what I'm looking for.
But I don't want to build it from scratch. The complexity of my scooter rebuild is already off the scale.

Looks like solar charge controllers are a fair approximation of what I need.

What is the main pack voltage?
How much current do you need to supply on the 12v circuit?

They make inexpensive dc-dc converters that have adjustments for both voltage and current limit. If you can find one with the right ratings, it would work.

Main pack is 20s.
I don't need a lot of current at all. A CC/CV supply would be fine to slowly lift the line back to supply voltage without blowing up the DC-DC converter.

But honestly, those solar charge controller which drive the battery and load separately cost less than 10 bucks.
I think I will run a few tests with those.
A more suitable product along those lines could hardly cost more. Will see if I can find something still.

At 20s, you could probably find a switching mode lead acid battery charger made for 120vac that will work fine straight off the pack.

Here is one:
https://www.ebay.com/itm/12V-Smart-...024459&hash=item4668e0ea92:g:-NoAAOSwmU1Zk~oe

Another one:
https://www.ebay.com/itm/12V-1-2A-T...678789?hash=item3a95398845:g:5DcAAOSwu1VW4QJ~

Look for a charger that has 110v-240v input (no switch).

All the solar chargers have too high current for my puny little battery.

I will do it with discrete components. There doesn't seem to to be an out of the box solution for me I can just buy.
Shame it has to be like that. I don't have a lot of space for discrete components. And I have to cram it into a little project box for weatherproofing.

There will be a relay that disconnects the battery while charging and supplies the always-on bus from dc-dc in the meanwhile.
And a constant current driver with DC boost stage (for LED) that will charge the battery. I will set it to 14V and 1A.

If I have to use a bigger Battery, I can always crank it up a notch up to 10A.