Having settled on a mixed-chemistry battery complement, selected new LiFePO4 batteries for the house bank and the various chargers and meters, the time comes to configure all the devices for charging and monitoring battery performance. Since no 2 of the components are from the same manufacturer, this is not a matter of referring to some standard support document, or even getting well-considered advice from knowlegable users. Herewith, the charger configuration I'm using, and a bit of justification for those choices.

The backstory on this is that I started off with a from-scratch needs assessment, hoping to be able to derive the charger settings from a first principles. I came up with some good scenaBut what I was specifying was "modal": I was imagining an "idling" mode that would keep the battery less than fully charged when the boat is sitting in the slip; and a "top off and go" mode that would charge the battery fully (before setting out) and would and trying to reason forward from that to the necessary charger configuration. What I learned along the way is I don't have enough flexibility in the configuration of the chargers (that I acquired) to actually get what I want that way.

But I did do some good thinking about scenarios and needs (retained below for your amusement) and I convinced myself I can do no better than others before me: optimize each of the chargers for a different operational regime and rely on the charger electronics to protect each from the excesses of the other.

accomplish that the design. falling back on first principles to design the configuration and convince myself it was feasible and perhaps even good.

Inventory:

• starting batteries: 2 x 100 Ah AGM batteries that have been in the boat for several years at least.
• stock 35 A alternator on 18hp Yanmar 2GM20F diesel. I chose not to invest in an alternator upgrade.
• house bank: splurged on LiFePO4: 2 x 206Ah batteries from SOK industries. These batteries consist of (industry standard) prismatic auto cells with a (field-upgradable) monitorable BMS all packaged in a nice waterproof plastic case.
• MasterVolt Combimaster 12/3000/160 inverter charger
• Renogy Dual Input DC-DC charger w/ MPPT
• Victron BMS 712 SOC meter
• Instrumentation Network -- NMEA 2000 backbone, RasPi server for SignalK, on-board WiFi and possibly an always-on internet portal. Navigation, wind and water instruments plus Gauges and instruments are mostly N2K (proprietary RayMarine), but bilge monitor is on SK and all data streams are visible on SK.
• . . .

Goals

What do I want this stuff to do for me?

• Don't force the battery into awkward places in its charge profile when it's just sitting in the slip. When it's in the slip, the battery does not have to be fully charged. LiPO is happy spending long periods at, e.g 50% SOC and is profoundly unhappy being floated at 100%.
• Do allow the battery to get topped up to 100% just before heading out, to get maximum endurance away from shore power.
• Under way, depend primarily on solar power to offset and replenish the day's consumption, augmented by occasional alternator. But don't expect these to completely replenish the batteries each 24h, instead expect to dip deeper into battery capacity each day. Some future boat may have enough solar and alternator to keep up each day.
• Design it such that alternator (maybe plus solar) generates enough to run all the nav and maintenance loads (though no cooking loads), with enough left over to net add some charge to the batteries. That is, if I've drawn the house bank down "too" far, I can move the boat without drawing it further down (assuming I can get the engine started from the starter bank).*
• I want reliable monitoring with early warning if I'm getting close to any of the edges of the performance envelope.
• Be reliable, unobtrusive and automatic. (Beautiful, cheap and quick).

Scenarios and load profiles

• Boat in the slip, solar and shore power charging. Draws are the maintenance items: bilge pump, network and non-sailing instruments. Goal: maintain LiPO at IDLING SOC where it may not have full capacity, but it will have the longest lifetime.
• Boat on the hook, solar and occasional alternator charging. Draws are the maintenance items plus living aboard items: lights, cooking, entertainment. Radios (VHF and AIS). A key metric is how many days I can go like this.
• Boat on the move, mostly sailing but motoring sometimes. Solar and occasional diesel charging. Draws are everything: all instruments, nav lights, radios and living aboard.

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When in the slip, the battery is idling, and it's important to prevent the battery from sitting at 100% (or 0%) SOC for very long.

LiPO battery operating regimes

IDLING : battery hovers around medium SOC for maximum life and cycle count. e.g in-the-slip scenario, must support maintenance loads indefinitely.

To support this, chargers would ideally charge the battery to around 60-75% SOC then stop charging until the battery discharges to 30-40% before repeating. LiPO does not like being kept at nearly 100%, so no traditional "float" charge phase.

The other regimes require the chargers to (attempt to) charge all the way to 100% SOC. It's not clear how to implement a mode setting to toggle the chargers between charge profiles. So if all we can doTo support this, the chargers should be configured to charge the battery but stop well short of 100%

Inverter / charger and DCDC charger (solar) cooperatively configured.

TOP-UP : battery brought to 100% SOC in anticipation of starting a discharge soon. e.g top up in the slip for an hour or 2 (or maybe the night before) in preparation for heading out. Must support maintenance loads while charging.

Inverter / charger is the primary source. How can we arrange a second profile available at the push of a button? Maybe diesel is involved?

DEEP CYCLE : battery being drawn down and simultaneously recharged as much as practical during on-the-move and on-the-hook scenarios. Won't start the next day as full as the day before till return to shore power or multiple days at rest on the hook.

DCDC Solar is the primary charger here, with incidential diesel (e.g, don't want to have to run diesel strictly for charging, but benefit from it when motoring)

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Design

[[ Calder said it better somewhere. ]]

Chargers have the characteristic that they are isolated from each other: big charging flows from each charger will go to the batteries and the DC loads but not into each other because diodes.

Sources and loads negotiate the current and voltage at the positive bus bar. It's Ohm's Law. So one charger that's in bulk charge mode and another that's in float can be tied to a battery that's at 50% SOC and you'll measure just one voltage at the bus bar.

This is an imperfect way of justifying the approach I'm going to take with charger configuration: configure each charger for the profile you want it to execute ignoring the presence of the other charger. Expect the battery voltage to tend toward the highest voltage that any one of the chargers currently "wants". If one charger is trying to float the battery at 13.8 right now, it will not "drag down" another charger that's trying to do a boost charge at 14.4. However, chargers are current-limited sources; they generally take time to get the battery

Monitors and Alarms

AGM too high (V) AGM too low (V) House ... House ...

LiPO combined charge

Bulk: constant current, from whatever low state of charge to voltage of 14.4, which indicates 90% SOC.

If already there, skip it.

Absorption (boost): constant voltage at 14.4 till charge current tails off to ((something corresponding to 90% SOC))

Float: skip it as much as possible. Let battery self-discharge or discharge under load to ((something corresponding to 50% SOC)), then start boost.

Charger Configuration

Inverter Charger

IDLING and TOP-UP regimes.

DCDC charger