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Buying New Marine Batteries Part II

In Part One, we reviewed the situation boaters face when they “just go and buy new marine batteries.”  The batteries you purchase may not be fresh, manufacturers do not indicate “sell by” dates, the products do not come with readily accessible owners’ manuals, and it may be difficult to return a dud purchased outside of the Caribbean.  In Brazil they have an expression that accurately describes this status quo:  “explica mas não justifica” (“it can be explained, but that does not make it right.”)  You need good batteries. Over and out! 

What can you do?  First, try to find a retailer who will exchange any batteries that you conclude are duds within say three to four weeks without arguments or hassle.  Do not accept any proposed verification of your conclusion by the retailer with a test device for starter batteries (which basically only measures the ability of the battery to supply a high current for a short time—what any fifty dollar starter battery can do).

Bear in mind that there may be significant differences between US and European battery dimensions and terminals. Finding out too late that the new batteries don’t fit in their box or having to change the cabling and/or terminals may not quite be what you had in mind, so check it out before deciding what to do and how to do it.

When you buy the batteries, check them for any mechanical damage. Then measure their zero-load voltage. Reject any 12v battery that is below 12.2 volts and any battery that shows a conspicuously lower voltage compared to its peers.

When you install your new batteries, buy or borrow a digital “clamp meter,” a plier-like clip-on current meter. It must at least be able to measure DC in the range of 0-100A, more if your alternators put out more.  A 300/1000 amp DC Velleman meter is very nice and costs approximately $150 US.  This meter clips around the cable that carries the current to or from a specific battery.

You no doubt already have a digital multi meter for measuring the voltage. (A multi meter cannot usually measure currents above 10 amps and would also cause an inaccuracy in the current measurement.)

Check that your batteries, looking to their alternator or charger, all see roughly the same cable resistance and that all connectors are clean, tight, and in good shape.

Carefully set the required voltages on your regulators and make sure temperature compensation is activated for gel batteries.  It is imperative that your regulators see the battery voltage on the battery, i.e. the battery voltage sense leads of the regulator must be connected directly to the battery terminals, via a small (e.g. 3A) fuse close to the battery.

Here in the tropics, operate the batteries in as cool an environment as you can achieve. Charge and discharge the new batteries a couple of times before drawing any conclusions. Although this apparently is necessary, manufacturers don’t as a rule tell you about it.  Using the clip-on ammeter, measure a couple of times what current each battery is supplying and absorbing during discharging and charging.

Use the digital multi meter to define the lowest and highest voltages of the battery bank. Bear in mind that a “no-load” voltage of 12 volts indicates for all practical purposes that the battery bank is discharged and should be recharged. (The typical voltage under load at that point could then be 11.6-11.8 volts.) Voltages lower than say 11.5 volts under load could cause appliances to disconnect, malfunction or be damaged. Remember that the deeper a battery is discharged on a regular basis, the shorter its lifetime.

Check that the highest voltage always remains under the maximum permissible voltage (for that particular type of battery), less the required voltage reduction for a higher than 25C battery temperature. Note also what the absorption and float charge voltage are that the regulators pursue. Put all these values in a table and see if they make sense.

A dud battery normally will give low current readings for both the charge and discharge cycles. If the readings remain low (compared to its peers) after a few cycles, return the battery and get a better one

Finally: it would be nice if you could check the temperature of each battery with a digital temperature meter (approx. 50-120 Euro) but using the back of your hand, it is easy to spot any cooler or warmer battery in the bank. If one of the batteries is either cooler or warmer, it is a bad sign. It is, however, normal that the temperature of the batteries increases by some 3-7 degrees Centigrade when relatively large currents run through them, either while charging or discharging. Of course this depends a lot on the currents vs. the capacity/physical size of the battery and also a little on ventilation: batteries in a tight box get warmer faster and stay warm for a longer time.

A final advice: don’t ever let your batteries sit in a discharged state for a long time, it will kill them!  If that would be unavoidable, then get good gel batteries.

John van Logchem is an electrical engineer reporting from Bonaire who has owned and sailed Queen of Hearts, a Swan 47, since 1992.

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