10A current source; Vin=14.5V, Vout=12.5V

[Myster E]

I'd like to ask you in the EEVblog community for thoughts, opinions, advice, etc... on building a 10A current source for charging a deep cycle battery from my vehicle's alternator. Common alternator output is about 14.5 V, and the battery's voltage while under a 10A charge would range from 12.5 to 14.5 V, increasing with state-of-charge (SOC) from 20 to about 90%.

I'm planning to replace my van's starter battery with a deep cycle battery (group size 24; ~ 80Ah). I need to limit the charge current to 10A, in line with the recommended max bulk charge rate of 0.13C for flooded batteries. This also saves on alternator stress.


Here's what I've been considering:

option 1 - do nothing: just swap batteries and move on

option 2 - off-the-shelf device: a DC-DC charger? probably out of my price range... and probably overkill...
I'm happy with the alternator's constant voltage function; just need to fill in the constant current part

option 3 - linear regulation: sounds barbaric... but is it?

option 4 - switch mode regulation: that's out of my league right now... but I'm up for it if it's the way to go

option 5 - pulsed regulation: ... like a lot of solar chargers do it?

option X - anything outside of my box sofar...

I have a few opinions on these options, but I'll save them while I'm listening...
What do you think?

[DenzilPenberthy]

Honestly, this doesn't sound like a good idea for a number of reasons.

- your starter battery is much better suited to cranking the engine than a deep cycle battery
- if you somehow limit the alternator output to 10A, that probably won't even cover the vehicle's consumption. Let alone once you turn on the headlights, windscreen wipers and blower motor. Your battery will run flat.

If you want a deep cycle battery in your van for some reason like using an inverter, then leave the starter battery and alternator as it is. Fit a deep cycle as a second 'leisure battery' and charge it from the vehicle through a 'split charge' system.

Cheap version is a voltage sensitive relay which connects the leisure battery when the alternator is running.
e.g. https://www.12voltplanet.co.uk/split-charge-relays.html

Better version, especially if you want to limit charge current or have a new vehicle with a 'smart' alternator, is use a DC-DC converter. e.g:
https://www.12voltplanet.co.uk/battery-to-battery-chargers.html

[Myster E]

Many thanks for your post Denzil.

Perhaps I should share a bigger picture of my plan:

Parallel to the battery there is an engine start module (ESM) - a scaled-down version of this https://www.maxwell.com/products/esm/.
It's a bank of supercapacitors (240F x 15V) with a balance circuit. It does the heavy lifting during starts. It lends a hand during peak surges - hard braking, steering ...
I'll install that first next to my current starter battery, which has been struggling lately. It should extend said battery's useful life. There's a handful of YouTubers that have done similarly for their vehicles.

The starter battery will be replaced with a deep cycle battery. I'll continue to rely primarily on the ESM for starting duties. In the worst case the ESM should start the vehicle after allowing the surface charge of a flattened battery recover to 11V or so.

Now, I want to spare the alternator from routinely recharging a battery from a 50% or even 20% state-of-charge (SOC). This would also be of concern with the conventional leisure battery + isolator arrangement, as you suggested. The vehicle's charging system isn't suited for this. Hence my wish for a current regulator. It will limit current into the battery (iow it's a charge controller); it will not otherwise hinder alternator output.

Lastly, the plan calls for an ideal diode circuit (https://www.ti.com/product/LM74610-Q1) bypassing the current regulator; it allows current to pass (virtually) unhindered out of the battery. Hence, the vehicle's power sources are: 1) the alternator for steady-state power, 2) the battery through the ideal diode for intermediate-term power, and 3) the supercap bank for short-term power.

Let's start another thread should this plan attract further discussion.

... Now, about the current source:
Picture a conventionally isolated auxiliary battery charged from the vehicle alternator. I would still want a current regulator to limit bulk charging current... for the sake of the battery and of the alternator. In this topology I would call it a "current-limited battery isolator". I figure I don't need a DC-DC solution, since there's nothing 'smart' about my '92 Astro van's alternator  , and since resistive losses should be minimal with the battery housed under the hood (GB-EN: 'bonnet'  )... current regulator dropout losses notwithstanding.

cheers
Mr. E

[james_s]

The typical way to handle multiple batteries in things like RVs and boats is to use a battery isolator which is nothing more than a pair of heavy duty diodes in a housing. My dad's sailboat has a pair of large deep cycle batteries and there is a manual switch that lets you select Off-1-2-Both. IIRC it is a make-before-break type so it doesn't let the alternator run unloaded as you switch from one configuration to another.

You want to make sure that you're not limiting the output of the alternator to power other accessories, some of those can draw quite a lot. If it has an electric primary cooling fan that can pull 50+ amps. The HVAC blower can pull another 20-30 amps, same with the electric rear window defogger, headlights are 10-12A, it all adds up.

If you do decide to build some kind of regulator, make sure you take into account the fact that an automotive electrical system is notoriously dirty. You can get all kinds of spikes well beyond the nominal 12V, and it can dip down as low as 6V or so cranking the engine on a very cold day.

[Myster E]

Thanks for your post James.

james_s wrote:

The typical way to handle multiple batteries in things like RVs and boats...

james_s wrote:

You want to make sure that you're not limiting the output of the alternator to power other accessories, some of those can draw quite a lot. If it has an electric primary cooling fan that can pull 50+ amps. The HVAC blower can pull another 20-30 amps, same with the electric rear window defogger, headlights are 10-12A, it all adds up.

I've moved and responded to these comments in a new thread: "Replacing starter battery with deep cycle battery" https://www.eevblog.com/forum/renewable-energy/replacing-starter-battery-with-deep-cycle-battery/

james_s wrote:

If you do decide to build some kind of regulator, make sure you take into account the fact that an automotive electrical system is notoriously dirty. You can get all kinds of spikes well beyond the nominal 12V, and it can dip down as low as 6V or so cranking the engine on a very cold day.

... not to mention reversed battery conntections (-12V), 24V jump starts, suppressed load dumps (up to 40V for a second or more), or even unsuppressed load dumps (100V+); a possibility in my crate, a '92 Astro van.

I've already designed a front end for any regulator I will build. It covers:

• electrostatic discharge (ESD) and other spikes
• unsuppressed load dumps (I have no idea if my vehicle has load dump suppression)
• negative input (supercaps reversed)
• negative output (battery reversed)
• overvoltage (lockout at 16V)
• input filter for regulator circuits