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Compass Marine How To | all galleries >> Welcome To >> LiFePO4 Batteries - Thoughts & Musings > AC Battery Chargers
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AC Battery Chargers

WARNING: LFP batteries should be OFF-LINE while dock-side and in storage mode preferably at 50-60% SOC. Your expensive LPF bank should not be float charged at the dock. This is really, really bad for them. Please wire your system so a lead acid battery can take over while you are at the dock.

Just like with external regulators we want to be able to control;

Max Current Output - A small generator may not be able to handle a large charger

All Charging Parameters - This is a must for an LFP bank

We also want;

Dedicated Voltage Sensing - This is a critically important aspect for an LFP design.

Large Output Capability - This compliments the bank and allows for faster charging. The largest charging capacity commonly found will be in a "combi" or inverter/chargers (I/C's). Stand alone chargers are usually smaller in current output. (NOTE: Most I/C's drop current output as they heat up. Just because it says 130A does not mean it can do this indefinitely. When hot you may see considerably less than "rated" output.

Dedicated Voltage Sensing Rant:

Here we go again.. In a nut shell there are very, very few chargers or inverter chargers out there that offer dedicated voltage sensing. This is STOOPID! Sadly the charging portion of most inverter chargers is apparently after thought for most engineers who design them. The engineers who fail to provide dedicated voltage sense leads on battery chargers or inverter chargers have failed you as a customer who is actually looking for fast charging performance..

Who are these failures?

*Mastervolt (no dedicated voltage sensing)

*Magnum (no dedicated voltage sensing)

*Xantrex (no dedicated voltage sensing)

Which manufacturers actually care about battery charging performance?

*Victron (Victron I/C's have dedicated voltage sense terminals right on the main unit)

Outback (NOTE: Requires FLEXNET DC & MATE Remote Control)

This is not to say Victron makes the best inverter chargers but god damn if their engineers actually get it. Battery charging that is. (wink)

Follow me on this. No matter how big you size the wire for with a 130A+ inverter/charger you will still have some voltage drop between the charger and the physical battery terminals. Most charger manuals only account for wiring voltage drop but we should remember that each termination, busbar, shunt, fuse, battery switch etc. results in even more voltage drop. It is not uncommon to see 0.4V -0.8V of drop, at full charging output, even on factory installed inverter/chargers. While the I/C makers often insist you keep the unit 5' from the batteries, this is not always possible on a boat. In the real world voltage drop happens and is simply a fact of life.

How do you fix that? Simple, dedicated non current carrying voltage sensing leads connected directly to the battery terminals so the charger can compensate for slight voltage drops in the system wiring and not enter absorption or voltage limiting prematurely..

Consider that just a 3% voltage drop, something most boat owners feel is perfectly acceptable, winds up creating a .42V drop, at the battery terminals, when the charger is pumping maximum amperage and trying to attain 14.0V.

If we start with a target voltage of 14.0V, and drop or lose .42V, this means just 13.58V at the battery terminals. Even an LFP bank, which normally operates around 13.2V, can come up to 13.6V well before it should have its current limited by the charge source.

Due to voltage drop the charger simply begins limiting voltage because it thinks the batteries are at 14.0V, but they are not. The charger is thinking it's in constant voltage mode but it should not be. This type of situation literally murders fast charging performance. This is not just on LFP banks but also on large lead acid banks too. of course because LFP has such a narrow voltage window that it operates within system voltage drop becomes a larger issue, performance wise.

How this process occurs:

At the charger end it sees 14.0V and enters CV mode or constant voltage mode. It now begins limiting current by controlling the output of the power supply, so as to not over shoot 14.0V. The problem is at the battery end the voltage is below 13.6V and only so much current can flow into the battery at 13.6V, even an LFP bank. Voltage is the pressure that allows the charge current to flow into the batteries and LFP banks are not Ohm's law exempt.. Attain a "limiting voltage" too early, due to voltage drop, and you have just extended your charging times.

Voltage sensing at the battery terminals is critical to FAST CHARGING PERFORMANCE. If you use a generator to power an AC charger proper voltage sensing means less generator run time. If a charger or inverter charger does not offer you this option BUY ONE THAT DOES!

The Victron's represent an excellent value in an LFP capable inverter/charger, especially one that has dedicated voltage sensing leads. By the time you are done with the Outback, by adding FLEXNET DC (allows for volt sensing) and the MATE (remote control), you are well in excess of the cost of a Victron Multi-Plus I/C. Course if you are in the US the Outback is a US company and supports US jobs.....

Choose your AC charger carefully. The two biggies are full control over all charging parameters and dedicated voltage sensing!

other sizes: small medium large original auto
Graeme 14-Sep-2015 22:44
I'd like to correct the statements about Mastervolt chargers and voltage sensing. Mastervolt have two main series of chargers: "Mass" and "ChargeMaster". The Mass series does provide direct voltage sensing. The ChargeMasters do not.

Also, Mastervolt expect a complete, large system to employ a "MasterShunt". Apart from measuring net current into the battery bank (charge current minus load current), the MasterShunt provides for voltage and temperature sensing of the battery bank. Both the Mass and ChargeMaster series (except the smallest) accept data from the MasterShunt over the MasterBus (canbus-based) data link, to provide voltage compensation.

In addition, by using the MasterShunt, you are providing the charger with the true current charging the battery, for correct determination of the charge endpoint based on absorption charge current (more relevant for lead acid batteries). Chargers that us their own output current to estimate the battery current can be fooled by a large steady load on the bank during charge.

(I'm not a Mastervolt dealer, but I do have both Mastervolt and Victron equipment on my boat.)