Making the Most of Bank Charges

We’re still on the electrical trail this week, but I promise I’ll have some more entertaining stuff for you soon. Mind you, electricity can get quite entertaining if you don’t do things correctly, just ask this guy!

A conundrum cropped up while helping customers plan the change-over from lead-acid batteries to a RELiON lithium battery bank, and we’ll get to that soon. But first let’s review current practices for charging multiple battery banks.

Charging Diodes. Many vessels have separate battery banks for the house supply, engine start, thruster, etc. Shore power chargers typically have multiple outputs, each isolated from the other so that separate banks can be charged simultaneously, but what about charging those banks from the alternator?

That’s where Charging Diodes come in. These are essentially a one-way valve for DC current, thereby allowing a charge current from the alternator to charge various battery banks but then not allow any back or cross-feeding of the banks when the alternator is idle. The issue with typical Charging Diodes is that they produce a reduction of the charging voltage across them, typically 0.7v to 1.0v, and so some batteries would be at risk of not being properly charged.

Remote voltage sensing is available for some alternator regulators, but only for one battery bank, so the other banks can suffer a charging voltage deficit. Charging Diodes are also used with Inverter/Chargers, as these items have a single charging output and have replaced multiple-output shore power chargers in many installations.

Voltage Sensing Battery Combiner. This is essentially a voltage sensing relay that connects two battery banks when a charging voltage is detected, and then disconnects when the voltage drops below that expected from a charging source. In 12v systems, the two battery banks will be connected in parallel when the Combiner sees a voltage of around 13.4v to 13.8v and will then isolate one from the other when the voltage drops to around 13.0v, which would indicate that there is no charging taking place.

Battery combiners come in various forms and have different names. Some are capable of charging just one auxiliary bank, while others can connect two or three auxiliary banks to the main bank. There are Combiners that can handle just small charge currents and others that are capable of safely handling up to 500 amps.

Some are actual mechanical relay switches and others are solid state MOS-FET devices, but all Combiners work by using a voltage threshold as a means of telling when a charge source is operating.

Battery-to-Battery Charger. This is a fairly recent development, and not yet seen in widespread use. It still employs voltage sensing to know when to combine and separate two battery banks, but in addition the auxiliary bank will be charged with a multi-step program and at voltages that are completely independent from to whatever the charger or alternator is charging the main bank. This ensures that the auxiliary battery is correctly charged, especially if it is of a different type and/or chemistry than the main bank.

Now, back to the conundrum. A standard lead acid cell measures a nominal 2.1 volts when fully charged, so there are six of them wired in series inside a 12v battery. Most lead-acid batteries will measure 12.6 to 12.8 volts when fully charged and rested for 24 hours, and some even up to 13.0 volts.

But LiFePO4 cells measure a nominal 3.2v when fully charged, so we have just four of them wired in series to make a 12v battery. RELiON uses high-quality cells in their LiFePO4 batteries which results in a slightly higher cell voltage, and a fully charged RELiON 12v battery will measure 13.2v-13.3v at rest.

Now, if you look at the connect and disconnect voltages for various Combiners and Battery-to-Battery Chargers, it will be seen that these devices will indeed connect a main RELiON LiFePO4 bank and an auxiliary one if a charging voltage is detected. But then the voltage threshold required for the batteries to be separated again when charging stops is below the at-rest voltage of the RELiON bank.

The result would be that the main and auxiliary banks would be almost permanently combined, with or without a charger operating, and while that may not necessarily be a bad thing, the Combiner is not doing the job it was installed to do. Ideally, a Combiner on a RELiON LiFePO4 bank would need to have a cut-in voltage of 13.6 or higher, and a disconnect voltage of 13.4v.

Most of the Combiners and Battery-to-Battery Chargers I have looked at do not have adjustable cut-in and disconnect voltages, and those I’ve seen that are adjustable don’t seem to have the range suitable for a RELiON bank. There is one that seems to tick all the boxes, and that’s the ME-SBC from Magnum Energy. This has a suitable, adjustable voltage range and a current rating of 25 amps, with provision to operate an external relay/contactor if larger currents are anticipated. Looks good to me!