Keeping a start battery charged

[a64pilot]

Originally Posted by BlackHeron

Charging a battery is not like filling up a glass of water. Charge current is driven by voltage and resistance. With the larger more-discharged house bank dropping circuit voltage down to bulk or acceptance charge levels the voltage will never get to.levels where it can ever push the start battery into equalization, its internal resistance simply will not allow it to offgas or overcharge until voltages get higher. This is basic DC series/parallel theory.

That is all lead acid batteries, so if that’s correct like I said then we should all just run everything at absorption voltage.
I understand battery acceptance rate, which is what your talking about, however the higher voltage we push at absorption WILL overcharge a battery, WILL cause it to off gas which deletes electrolyte and an AGM only has the amount it came with, so every single day, day in and day out pushing absorption voltages to a fully charged battery will shorten its life.

If what your saying is correct, then why do we have three stage chargers?

Automotive voltage standard used to be 14V plus or minus .2V so 13.8 to 14.2
Not higher than 14.2, I assume it’s the same.
Automobile alternators are not meant to charge batteries, they are meant to run the electrical system, if you want to charge a battery, you need a higher voltage that cuts back when the battery is full or you will overcharge, that’s called absorption voltage and is why our three stage chargers exist.

I may do the ACR as with the length of cable it’s likely there will be enough voltage drop to get me closer to 14V and that should be low enough for the Odyssey as it’s an automotive battery anyway.
However a battery tender gives me automatic three stage charging for $20 or so.

But also it will keep the little Odyssey from ever going to equalization voltages, which it’s not supposed to see, because if the ACT combines the banks, and it will, and if the 660AH bank is at equalization voltage, and it will be, then so will the little Odyssey.

[BlackHeron]

I see somebody posted a link to you earlier that went over the science much better busting the myth of overcharging banks in parallel. If you didn't read or understand it there is no point in my going over it all again.

Believe whatever you want, the engineering and scientific principles behind that do not care what you believe or don't believe. Do what you want on your boat, I just don't get why you ask for advice and then ignore it because you are so sure you know better.

[a64pilot]

I did read and understand it, but it’s wrong plain and simple.
Putting a battery through an absorption voltage charge day after day with it being at 100% SOC will overcharge it. That article, said that a battery when fully charged simply won’t take any more current, well if that’s true, then why float a battery? It’s not current but voltage that will over time harm the battery. At equalization voltsge my 660 AH bank only accepts about 2 amps, but leave one at equalization voltage and you will kill a bank.
It will at the least consume more electrolyte meaning if it’s flooded you will be adding water frequently, and if it’s an AGM, which these are, it will dry out the glass mat, they aren’t 100% efficient in recombining gasses, nothing is 100%.
Automobiles don’t or shouldn’t operate at absorption voltages, if they did, then a simple automobile alternator would charge a bank just as fast as an expensive three stage charger., and guess what, they don’t. You raise voltage to absorption voltage to push the charge in faster, then back off to float to keep from overcharging the battery, otherwise why not stay at absorption?

I don’t know why you can’t understand that simple fact, it is pretty simple. I don’t think you can find a battery manufacturer that says that subjecting their battery every single day to several hours of absorption voltage with it being fully charged to begin with isn’t bad for it.
And that‘s what would happen with my Solar.

[BlackHeron]

I have a small group 34 start battery on an ACR in parallel with my house bank which is four 6v golf car batteries connected in series/parallel. They are all FLA chemistry on our boat and I check the electrolyte levels regularly. We add distilled water but it isn't excessive in my opinion or in my experience in maintaining battery-powered construction equipment such as scissors lifts.

Our start battery is now four years old and the GC2 batteries are all three. We hit equalization charge nearly every day when it is sunny, usually by noon or so.

Electrolyte level in the golf car batteries goes down much quicker than the start battery even though it is often not used to start the engine for months at a time, but when we are motoring we can run for 10-12 hours a day, charging the house bank with the alternator from a much lower state of charge when we start out in the morning. The engine rarely cranks for more than a second or two and pops right off without a hitch moat mornings. This start battery rarely gets much of a workout other than being equalized nearly every day.

When I check up on the specific gravity on all five of my FLA batteries they all are testing like brand new and as expected at their various states of charge. But what do I know? I was only paid to maintain FLA batteries at work for decades as part of my job running commercial electrical jobs. I guess I hardly know what I am talking about.

Everything I was taught in electrical school seems to line up exactly with my experience in practice with our two battery banks. Maybe physics just works differently in your world as mine seems to be "wrong."

[a64pilot]

I’m glad your having good luck with your batteries, Although it’s definitely overcharging it if I use the ACR, it’s likely abuse that the Odyssey can tolerate, my absorption voltage is 14.3, but as there is probably at least 20’ of 2ga round trip the voltage loss is likely to put the actual at the little battery voltage within its acceptable limits as my absorption is 14.3, measured I’m sure at the Solar controller.
14.2 is acceptable for an automobile, the Odyssey is an automotive battery, and that’s only .1V under, so I think it will tolerate it
So bottom line, it’s is abuse, it is overcharging, but likely it can tolerate it.

But for $20 I can have three stage charging, and save a lot of cable run. Cable is already there or it would be a no brainer.
I’ve used battery maintainers for years and been real happy with them.

[BlackHeron]

I'm glad Im lucky.

[Adelie]

Put a 10w panel in a window, run it thru a PWM controller and forget about it.

[Wotname]

Originally Posted by a64pilot

I did read and understand it, but it’s wrong plain and simple.
Putting a battery through an absorption voltage charge day after day with it being at 100% SOC will overcharge it. ........

This is completely true!!!!

When all else fails, read the manual (this comment is not directed to you a64pilot, rather it is directed to the wider CF readership).

The Odyssey service manual is very clear about the proper charging profile for their batteries.

Simply put, the recommended profile is bulk, then a 8 hour absorption charge at 14.7V, then float at 13.6V.

From the manual - these batteries have a design life of 10+ years in float applications; at end of life an ODYSSEY battery will still deliver 80% of its rated capacity.

These numbers are for 25C, more detail available in the manual.

[mitiempo]

Originally Posted by a64pilot

I did read and understand it, but it’s wrong plain and simple.
Putting a battery through an absorption voltage charge day after day with it being at 100% SOC will overcharge it. That article, said that a battery when fully charged simply won’t take any more current, well if that’s true, then why float a battery? It’s not current but voltage that will over time harm the battery. At equalization voltsge my 660 AH bank only accepts about 2 amps, but leave one at equalization voltage and you will kill a bank.
It will at the least consume more electrolyte meaning if it’s flooded you will be adding water frequently, and if it’s an AGM, which these are, it will dry out the glass mat, they aren’t 100% efficient in recombining gasses, nothing is 100%.
Automobiles don’t or shouldn’t operate at absorption voltages, if they did, then a simple automobile alternator would charge a bank just as fast as an expensive three stage charger., and guess what, they don’t. You raise voltage to absorption voltage to push the charge in faster, then back off to float to keep from overcharging the battery, otherwise why not stay at absorption?

I don’t know why you can’t understand that simple fact, it is pretty simple. I don’t think you can find a battery manufacturer that says that subjecting their battery every single day to several hours of absorption voltage with it being fully charged to begin with isn’t bad for it.
And that‘s what would happen with my Solar.

You realize that equalization voltage is 15.5 to 16 volts. We are not talking about that here though.

And a simple automobile alternator - similar to the stock alternator on most engines as delivered - if set at the same voltage as the bulk/absorption voltage of a 3 stage charger will charge the battery to full equally fast. Manufacturers/suppliers of 3 stage regulators don't tell you that. The 3 stage regulator has many other advantages but speed to full charge, assuming same amperage rating, isn't one of them.

[Wotname]

Originally Posted by a64pilot

........ my absorption voltage is 14.3, but as there is probably at least 20’ of 2ga round trip the voltage loss is likely to put the actual at the little battery voltage within its acceptable limits as my absorption is 14.3.......

I won't count on getting much voltage drop in wiring as you have described. The float current will be very very small and the voltage drop is a function of current and resistance.

The resistance of 20" of 2ga wire is quite low and the float current is very very low so the voltage drop will also be very very very low. I haven't run the numbers but I can if you wish.

FWIW, the float current of a PC1200 is 4 milliamps and if the resistance of the wire is 0.1 ohms (but 20' of 2ga will be much lower), the voltage drop would be 400 microvolts.

If your Odyssey is much smaller than the PC1200, the float current will lower again.

Gut feeling is we are talking in the order of tens of microvolts of voltage drop in the wiring!

[mitiempo]

Another point that should be mentioned. A multi-output battery charger from any major manufacturer - Victron, Promariner, Xantrex, Blue Seas, Mastervolt, etc - connected to both the house bank and the start battery, as is done on many boats, is doing exactly the same thing that an ACR is doing. When it is is bulk/absorption for hours charging the house bank the start battery sees the same voltage - 14.4 or so. Exactly the same as an ACR. This doesn't kill start batteries either.

[Wotname]

Originally Posted by mitiempo

Another point that should be mentioned. A multi-output battery charger from any major manufacturer - Victron, Promariner, Xantrex, Blue Seas, Mastervolt, etc - connected to both the house bank and the start battery, as is done on many boats, is doing exactly the same thing that an ACR is doing. When it is is bulk/absorption for hours charging the house bank the start battery sees the same voltage - 14.4 or so. Exactly the same as an ACR. This doesn't kill start batteries either.

There is one significant difference between the house bank and a generator start battery.

The house bank is continually being discharged and it rarely held at absorption levels while at 100% SOC for days at a time.

A generator start battery will spend most of it's life at a 100% SOC and should be held at float voltage during this extended time.

Car batteries are different again, they aren't held at absorption levels for long times, they have greater self discharge characteristics than the Odyssey (which this thread is about), they are cheap and are replaced regularly.

[sailcrazy]

We use LA Deep-Cycle (6-Trogan T105s) for our house bank, and use a single mx-free START batt(different plates for a different load than the house) for starting either the main eng or the genset. All our charge sources go to the house bank (solar, wind, shore power batt charger, and eng alternator), and we use a Balmar Duo-Charge from the house to the start batt to keep it(the start batt) recharged. I monitor both batt systems with our BMs, but other than that the Duo-Charge required no input or adjustment....and it's worked perfectly now for 5 years.

[mitiempo]

Originally Posted by Wotname

There is one significant difference between the house bank and a generator start battery.

The house bank is continually being discharged and it rarely held at absorption levels while at 100% SOC for days at a time.

A generator start battery will spend most of it's life at a 100% SOC and should be held at float voltage during this extended time.

Car batteries are different again, they aren't held at absorption levels for long times, they have greater self discharge characteristics than the Odyssey (which this thread is about), they are cheap and are replaced regularly.

It is the same situation as any large house bank and separate start battery boat - of which there are many thousands. Many with ACRs. Many years ago Blue Seas had sold 500,000 ACRs. By now quite a few more.

As an example my car battery is charged by an alternator set at 14.4 (measured) voltage and they last 8 to 10 years. That is a lot of charging at 14.4 volts.

Here's a link to Rod Collins on ACRs
https://marinehowto.com/automatic-charging-relays/

[noelex 77]

A64pilot is correct that in the circumstances described the ACR will overcharge the start battery.

Overcharged start batteries are quite common with many charging solutions. However, the start battery otherwise has an easy time and will likely have a reasonable life even with some overcharging. Given the low cost of the start battery, the overcharging is not a big deal and the other advantages of ACR (such as the foolproof nature of the switching) make it a good option.

Personally, I think the solution outlined by Wingsail is worth considering. We used the same system on our previous yacht for our engine start battery. It is the simplest, most reliable system (ACRs do occasionally fail) and allows the correct charge profile, maximising the start battery life, which helps reiability as well as reducing costs. It is also slightly easier to pick when the start battery is nearing the end of its life especially if you have a solar system. The surface charge from the ACR can mask the deterioration in the start battery performance.

If you do very occasionally forget and inadvertently leave the battery banks joined when discharging, it is extremely unlikely that this oversight would also be combined with the very rare occurrence of draining both banks to a level where they are unable to start the main engine. In A64pilot’s case dealing with a generator start battery the concern is even less.