Replacing starter battery with deep cycle battery

[Myster E]

I've spawned this thread from an earlier one about building a current regulator (https://www.eevblog.com/forum/renewable-energy/10a-current-source-vin14-5v-vout12-5v/).
That thread started out attracting more discussion about my reasons for building the regulator, rather than on building the regulator itself.

So... the subject of this thread is my plan for getting some coach power going in my Astro van.
This is the outline of my project:

I will replace my vehicle's starter battery with a deep cycle battery - group size 24; 80 Ah.
In parallel with this battery I will install a supercapacitor bank - 240F @ 15V; a scaled-down version of an Engine Start Module (ESM) like this: https://www.maxwell.com/products/esm/.
This combination will allow me to start the vehicle even when the deep cycle battery is at 20% state-of-charge (SOC)... without a starter battery in sight.

To avoid routinely overloading the alternator by recharging a depleted deep cycle battery, I will limit the charge current into the battery to 10A.
This also respects the recommended maximum bulk charging rate for flooded batteries of 0.13C (80Ah x 0.13 = 10.4A).
At the same time I will allow unhindered current out of the battery to satisfy vehicle power surge requirements: cooling fan, power steering, hard braking, etc...

The conventional method of providing auxiliary coach power is to install a deep cycle battery in addition to the starter battery, and using a battery isolator to avoid depleting the starter battery with coach loads. See https://en.wikipedia.org/wiki/Battery_isolator.
There is no space under my van's hood for a 2nd battery, and I want to avoid installing an auxiliary battery in the interior of the van.
I realize that I might quickly outgrow the 80Ah (or whatever I can fit onto the battery tray), but I want to do this as a proof-of-concept excercise.

I believe that it's a waste to haul around a big, heavy starter battery just for cranking the vehicle's engine.
I'd like to use all that energy for something useful... like cranking up the stereo and/or playing on the laptop. 

[Myster E]

This discussion was moved from thread "10A current source; Vin=14.5V, Vout=12.5V" https://www.eevblog.com/forum/renewable-energy/10a-current-source-vin14-5v-vout12-5v/

james_s wrote:

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

I'm thinking outside the box on this one, James.
I'm combining a deep cycle battery and supercapacitor bank to provide both starting and coach power.
The supercap bank is a scaled-down version of a commercial Engine Start Module, like this https://www.eevblog.com/forum/renewable-energy/10a-current-source-vin14-5v-vout12-5v/.
It's smaller and lighter than a starter battery would be.

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'm not limiting output of the alternator at all. I'm limiting charging input to the battery.
The alternator sees just another 10A accessory instead of a flat battery.
... and I'm not limiting the battery's ability to contribute to the vehicle's transient power demands.

[trobbins]

Perhaps keep your combustion engine system in good shape, and test how much the super cap depletes in voltage during a standard crank cycle.  If your engine doesn't start, then you may find that you have to wait quite a few minutes before you can crank again.

You may need to take in to account the self-discharge power of the supercap when you are not using the engine power to keep it 'topped up' - ie. have some form of manual disconnect for long periods.

[Myster E]

trobbins wrote:

Perhaps keep your combustion engine system in good shape,

Good advice on any given day, without a doubt.

...test how much the super cap depletes in voltage during a standard crank cycle.

0.5-1.0V per successful start for one 80F series. The installed bank will have 2 or 3 parallel series for 160-240F.
My target is ~5 sec of quality power starting at 12.0V

You may need to take in to account the self-discharge power of the supercap when you are not using the engine power to keep it 'topped up' - ie. have some form of manual disconnect for long periods.

...or a small dashboard PV panel to float the battery...
The caps themselves are pretty good; they were at 9V after more than a year in storage.
I'm tweeking the balance circuit to keep self-discharge in control. It's an active balancing op-amp design.
The individual ICs require sub-uA power, but I suspect they're shuffling charge when they should better be idle.
Once I confirm this I'll try replacing them with comparators; the hysteresis may settle things down

[Circlotron]

test how much the super cap depletes in voltage during a standard crank cycle.

I know that 60 Farads @ 14V will crank a 5 litre V8 about 3/4 of a turn... 

[trobbins]

Did you measure the 1V drop (ie. 16V to 15V) for a single successful crank start of your Astro van engine?

The self-discharge will be max when the cap is sitting at 16V.

Your first post indicates the supercap is in parallel with the battery.  But you refer to the ESM product (but a reduced capacity version).  Can you confirm the battery is effectively in parallel with the supercap B+ or charge terminal, and the starter motor circuit is across just the supercap cap output (ie. will vary from circa 16V down to whatever the starter motor will discharge it to if it were to keep cranking).

[Myster E]

trobbins wrote:

Did you measure the 1V drop (ie. 16V to 15V) for a single successful crank start of your Astro van engine?

I acquired and initially tested these supercaps several years ago, and I don't have that diary handy.
As I recall, I used a single 80F string equalized to the vehicle battery at rest (ie 12.5-ish V).
I usually got 2 successful starts bofore falling to 11V.
I could successfully start at 10V (just once before recharging back to battery voltage).
It was awesome to observe very little voltage sag during cranking.
I was convinced that 2 or 3 strings in parallel would be satisfactory for my vehicle.

The self-discharge will be max when the cap is sitting at 16V.

I suspect the caps have increased internal resistance since they were new.
As well, I might have heat-damaged one string while attempting to solder balance leads to the intermediate tabs.
The middle cell has increased leakage; it makes a good test case for the balance circuit.
I'll be doing renewed testing when I'm done with the balance circuit.

Btw these are series of 5 x 400F@3V cells = 80F@15V bank (Amperics Inc);
The datasheet for the discrete cells specified 3.1V absolute max while the datasheet for the 5X module specified 14V max; guess they left a little breathing space for cell imbalance.
These modules are rated a tad too low for alternator charging... but the price was right and they were adequate to get me going on a proof-of-concept.

Your first post indicates the supercap is in parallel with the battery.  But you refer to the ESM product (but a reduced capacity version).  Can you confirm the battery is effectively in parallel with the supercap B+ or charge terminal, and the starter motor circuit is across just the supercap cap output (ie. will vary from circa 16V down to whatever the starter motor will discharge it to if it were to keep cranking).

The Maxwell ESM is a 3-terminal device with B(attery), S(tarter) and G(round) terminals. The internal supercap bank and starter are isolated from the battery by (presumably) a boost charging circuit and backflow block, while the starter is hard-wired to the supercap bank. See 'Supercap Starter'/'Smart Start' (Fig 2) in https://www.eenewspower.com/content/extending-battery-life-transportation-and-mobile-applications-supercapacitors.

The 'ESM' in my topology is a 2-terminal device without supercap+starter isolation. It corresponds to the 'Direct Parallel" supercap installation method in the above article (Fig 1). That is simply the battery and supercap bank in parallel hardwired to the starter. As such, there is no boost charger to bring its voltage up to 16V, nor does it retain the full 14.4(+/-)V from the alternator. When the vehicle is parked the supercap bank voltage will track the battery's voltage as it settles to 12.5V at rest.

I made the comparison to the commercial product without intending this level of detail. I likened the two based on the broader concept of using supercaps to start a vehicle. Perhaps I overreached by referring to my supercap bank as an 'ESM'.

[trobbins]

Does that mean the battery and cap will discharge directly in parallel, and you will have some form of charger limiter for that batt/cap combination such that the batt charge current won't exceed its rated limit?

And the lighting/acc circuits are directly across the batt/cap?  Is there an LVD associated with those circuits?

The alternator is not working directly in to the battery - which may be a concern, as that voltage is not clamped by the battery.

How did you determine the 20% SoC lower limit for starter operation?  Is there an LVD or some form of inhibit (eg. starter solenoid drops out) if batt/cap voltage gets low?

[Myster E]

trobbins wrote:

Does that mean the battery and cap will discharge directly in parallel, and you will have some form of charger limiter for that batt/cap combination such that the batt charge current won't exceed its rated limit?

When the supercap bank is good to go I'll install it next to my current starter battery.
The battery's started struggling now and then and the supercaps should extend its useful service life.
Eventually I'll acquire a deep cycle battery to replace the starter battery.
I envision (partially) isolating that battery from the rest of the vehicle with a charge controller that regulates charge current to 10A but allows current to pass unhindered in the other direction.

And the lighting/acc circuits are directly across the batt/cap?  Is there an LVD associated with those circuits?

Battery over-discharge has not been a frequent problem for me sofar.
When I install the deep discharge battery I'll consider LVD to limit discharge due to coach activities.

The alternator is not working directly in to the battery - which may be a concern, as that voltage is not clamped by the battery.

... but there will be that supercap bank.
I remember doing calculations that led me to conclude that 80F worth of supercaps could absorb the mother of all load dumps with an increase of only a few millivolts. I don't recall how the calculation was done... maybe you can do the math.

I've also mused about designing the current regulator with very low bandwidth - a time constant in the order of seconds.
It's job after all is to regulate current to conditions that change slowly, to keep the battery from overheating; not to suppress transients.
In the worst case few seconds of badly regulated or even unregulated current won't do much harm.
For now I'm fishing for guidance on choosing the best basic technology for the regulator.

How did you determine the 20% SoC lower limit for starter operation?  Is there an LVD or some form of inhibit (eg. starter solenoid drops out) if batt/cap voltage gets low?

For the time being I'm using data from this article: https://www.scubaengineer.com/documents/lead_acid_battery_charging_graphs.pdf
... on the assumption that it's typical of flooded lead acid batteries.
It shows battery voltage vs DOD vs discharge rate.
So an 80%-discharged battery at rest should measure about 11.9V.
I can do my own calibration from specific gravity measurements once I've acquired a deep cycle battery.
I'm thinking of setting the LVD trip point at 50% SOC (12.3V at rest, or maybe just an even 12.0V) with a manual override to allow deeper discharge.