How to control charge voltage in multi chemistry system

[kas_1611]

I'm looking for a simple method to control the charge voltage from my Alternator to the 2 different chemistries in my battery system.

I have a 115A alternator that is split using a Sterling ProSplitR to provide charge to my 3 battery system. 2 of these (Starter and House) are standard Lead Acid sealed units, the 3rd (Thruster) is an AGM.

As the L/A units should have a charge voltage of 14.4V I am concerned that my AGM may well get damaged as it should have a max of 14.1V. Although the cable run to the AGM from the engine bay is long (approx 10m) it is a heavy duty cable so I am not sure there would be that much voltage drop.

The alternator senses the House battery voltage through the ProSplitR and all 3 battery units are standalone with individual on/off switches and fuses/breakers.

So the question to the collective grey matter is - what is the simplest way to ensure the AGM is not exposed to the higher voltage? Ideally without having to add in multiple extra bits of kit. I was thinking a voltage sensitive relay that would break the circuit at 14.2V but would this keep the battery correctly charged?

Many thanks for thoughts and ideas

Keiron

[Cheechako]

I bought a boat with existing Deca 4D GM batteries that were not new. I also had a wet starting battery. I never changed the output voltage on my regulator. (100 amp Ample power alternator and next gen regulator) I sold the boat 3+ years later and the GM batteries were still fine. Apparently the normal voltage didn't effect them at all.
Will yours perform like this? Hard to say. Normal charging the voltage doesn't stay high very long anyway does it?

[model 10]

A diode would have enough voltage drop. It would also serve a purpose.

[tkeithlu]

A voltage attenuator that could handle that amperage, if you could find it, would definitely add to your bits and pieces. It might be cheaper to go to all one type of battery.

[Fuss]

what is the voltage at the end of the cable... you might find that it's around 14.1 volts


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[smac999]

gel or AGM? AGM's charge at similar voltages as flooded. 14.4-14.6 should be good for agms.

AGM's and flooded batteries are both lead acid.

Gels are lower in the 14.1v range. and are not very common any more.

[kas_1611]

Thanks for all the replies so far

To clarify a couple of things that may not be clear in order to help the collective:

House and Starter batteries are sealed Lead Acid (L/A)
Thruster batteries (2 in parallel) are Optima Blue Top AGM

Alternator supply is split in 3 by a Sterling ProSplitR (PSR), zero volt splitter (rated to 180amps). This replaced the twin diode splitter system originally installed which was failing (giving high volt alarms on the engine control system). Alternator senses the House battery via the PSR.

House battery is monitored using a Nasa Battery Monitor. Normal charging voltage under engine is 14.4V to 14.5V. Checked and confirmed by multimeter. This is also seen at the Starter. I do need to verify the voltage seen at the Thruster (plan to install a voltmeter at some point).

I therefore agree that I may be worrying about nothing as the voltage at the AGM could be a couple of tenths of a Volt lower. As it turns out Optima themselves state the following charge voltages:

Alternator: 13.65 to 15.0 volts
Battery Charger: 13.8 to 15.0 volts; 10 amps maximum; 6-12 hours approximate
Float Charge: 13.2 to 13.8 volts; 1 amp maximum; (indefinite time at lower voltages)

Given this data perhaps there is nothing to worry about a 14.4V supply

Cheers and cold beers

Keiron

[Maine Sail]

Originally Posted by kas_1611

House and Starter batteries are sealed Lead Acid (L/A)
Thruster batteries (2 in parallel) are Optima Blue Top AGM

Your lead acid AGM Optima batteries will do best closer to 14.7V. Charging to just 14.1V would be under charging them.

"BLUETOP® Type: D34M / D31M/ D27M
These batteries are dual-purpose. They are designed for engine starting and deep cycling applications, as well as for use in boats with large accessory loads.
Cyclic Applications:
14.7 volts, no current limit as long as battery temperature remains below 125°F (51.7°C)."

[exMaggieDrum]

You might look in to a DC to DC charger, like the ones from Sterling. Some are made to step up or step down voltages between different volt systems, but you can get 12v to 12v. You can set the type of profile you want on the Sterlings. I have not looked at how "good" the charge profiles are for specific batteries but I suspect they would work in your situation.

Probably worth taking a look. I am considering one to charge my blue-top Optima start battery from a lithium bank, which has a completely different charge profile.

They aren't "cheap" but they are offered in various amp ratings which affects the price.

Here is one vendor's offer: Sterling Battery-to-Battery Charger - 12V-12V IP68 Waterproof

[Maine Sail]

Originally Posted by exMaggieDrum

I am considering one to charge my blue-top Optima start battery from a lithium bank, which has a completely different charge profile.

The Sterling B2B turns on at 13.3V and does not turn off until 13.0V, not a good fit for LFP unless you plan to manually control it. If you are going manual control a Balmar Duo will work fine and is less money. I simply installed a Firefly L/A reserve battery that is perfectly happy being charged at 13.8V - 13.9V and can sit there for a year or more with no loads and no charging and also be happy...

[exMaggieDrum]

Originally Posted by Maine Sail

The Sterling B2B turns on at 13.3V and does not turn off until 13.0V, not a good fit for LFP unless you plan to manually control it. If you are going manual control a Balmar Duo will work fine and is less money. I simply installed a Firefly L/A reserve battery that is perfectly happy being charged at 13.8V - 13.9V and can sit there for a year or more with no loads and no charging and also be happy...

Good comment MS. I haven't come up with an elegant solution, i.e. automatic, for the problem. Manual switching would work but always subject to remembering to turn it on and off at the right times.

I remember now that you had the Firefly. I really like them from all reports, although they are not cheap at $450 for a G31. Do they need a full AGM type "full" charge every once in a while even though they are happy at a PSOC for most of the time?

[kas_1611]

Well the good news is I have fitted a small voltage monitor to my thruster AGMs and the charge voltage from the Alternator is between 14.0v and 14.2v with the new heavy duty cables. This is an improvement of around 0.2v from the old, thinner cables.

If the engine is running for more than an hour the voltage drops back to 13.8v and fluctuates between 13.8v and 14.1v depending on the loading on the house battery (remember the alternator senses the House battery through the PSR). So this suggests that at least there is a good Float charge regime as per Optima's own advice
Alternator: 13.65 to 15.0 volts
Battery Charger: 13.8 to 15.0 volts; 10 amps maximum; 6-12 hours approximate
Float Charge: 13.2 to 13.8 volts; 1 amp maximum; (indefinite time at lower voltages)

As I kept the original cables in place when running the new ones I have the opportunity to hook up the 3rd port on the mains charger relatively simply. I will probably do this so that we have a controlled charge regime for all 3 battery banks anytime we are on the mains.

Admittedly the AGMs have been there for 5 years now, still hold a good charge for a long time and do the job when we need them. If I can keep them like that for a few more years then I'll be a happy bunny.

[boatbod]

If you only charge an AGM to 14.1 you will surely kill it from sulfation. Do yourself a favor and charge to at least 14.4 - 14.6, same as flooded.

[kas_1611]

Originally Posted by boatbod

If you only charge an AGM to 14.1 you will surely kill it from sulfation. Do yourself a favor and charge to at least 14.4 - 14.6, same as flooded.

This was my concern over the slightly under gauge cables and the reason why I fitted heavier gauge ones this summer. Well not the sulphation per se but the batteries not getting a proper charge from the engine, especially on those days when we only use the diesel lump for a few minutes as we come into a harbour.

As I am now getting a decent 14.2v to the Thruster AGMs, which is comparable to the voltage going to the 2 lots of sealed Lead Acids I am not so concerned. The AGMs have survived 5 years of abuse and are still going strong. As Optima themselves quote 13.65v to 15.0v from an Alternator the 14.2v I am getting now is slap in the middle.

Hopefully I can connect the mains charge to the AGMs in due course and these will get a proper charge when we are on the mains and enhancing their lifespan even further.

Cheers and cold beers

Keiron

[zboss]

We use an echocharger by xantrex but there are a few manufacturers.

All charging first goes to our lead acid batteries then to our agm starter battery through the echocharge. The echocharge limits the voltage to the voltage required by the agm battery and uses excess current from the main bank to charge the agm battery.

Simple and it works splendidly.