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Deficit reduction

Dec 16, 2013

More capable batteries with faster recharge rates can lower a battery bank’s charge deficit

(page 2 of 2)

Alternator heating
We knew that in any state of charge below even a moderately discharged state, the high charge acceptance rate of the TPPL batteries would drive the alternator to full output for an extended period of time. Even high output alternators rated for high temperature operation do not like this, and will potentially burn up, so we used the ‘belt load’ function in the Balmar regulator to limit the maximum alternator output to 80 percent (i.e., a nominal 96 amps).

Using a meter and a handheld probe, Calder determined that an internal sensor was cutting output when the alternator reached a temperature of 226° F.

As expected, the alternator rapidly ramped up to 90+ amps and remained at this level. However, within 10 to 15 minutes the charge rate dropped in half for a couple of minutes and then ramped back up. Thereafter it cycled about 50 percent of the time at high output and 50 percent at half. I knew what was going on. The Balmar regulator has an alternator temperature sensing device which is factory set to 108° C (226° F). If this temperature is reached, it cuts the alternator’s output in half. A quick check with a multimeter and thermocouple confirmed that at high output the temperature was rapidly rising to the setpoint. It should be noted that the ambient temperature in the boat’s cabin was around 80° F, which is certainly not unusual for a boat in a warm or tropical climate.

The underlying problem here is the chronic inefficiency of alternators. At peak efficiency they are rarely much above 50 percent efficient, and over many parts of their operating cycle they are well below 50 percent efficient. Taking our alternator as an example, at 90 amps output and a charging voltage of 14 volts, the alternator is putting out 90 x 14 = 1,260 watts = 1.26 kW of power. At 50 percent efficiency, it is also generating 1.26 kW of heat, which has to be removed from the case in order to prevent the windings and diodes from frying. Regardless of their rating, without excellent heat dissipation high-output alternators cannot be run continuously at anywhere near their full rated output.

Heat issues are exacerbated by the need to muffle the sound of the engine. In common with many boatbuilders, Malo installs excellent sound insulation in its engine compartments, but one of the inevitable side effects is to limit the airflow through the engine and machinery spaces. Any alternators, more-or-less wherever they are mounted on the engine, end up in a cramped space close to the insulation.

Extra cooling
Chris has a factory-installed engine room blower on his boat, designed to suck heat out of the engine room. There is a passive four-inch duct from the aft end of the cockpit coaming into the engine room to allow air in. We added four-inch ducting to the blower’s inlet side and led this to a point directly behind the alternator so we could suck heat right off the back of the alternator. In spite of a lot of hot air coming out of the blower’s exhaust vent, the alternator still overheated in about the same time as before, which was puzzling. I contacted Balmar for their thoughts and found out that this alternator has dual fans — one at each end — which suck air in from the front and back and exhaust it out the sides. By installing the ducting behind the alternator, we were effectively stalling the air flow at this end of the alternator, exacerbating the heating issues!

Nada’s old 120-amp, 24-volt alternator alongside the new Balmar 97 EHD 195-amp, 24-volt alternator. A big difference in scale!

We moved the ducting to a point where the blower sucked off the side of the alternator. From a cold start, the alternator now ran twice as long as previously before it reached the high temperature threshold, and then tripped to the reduced output level for about one-third of the time, returning to high output for two-thirds of the time. We made one final adjustment in the Balmar regulator, which was to raise the alternator’s high temperature threshold from 108° C to 114° C (237° F). Balmar states that this threshold can be raised to as high as 120° C (248° F) but we did not want to push the alternator to its limits. In any event, at 114° C, in an ambient cabin temperature of 80° F, and with the engine room blower sucking off the side of the alternator, the system now runs continuously without reaching the temperature threshold.

The net effect of these changes is to substantially reduce the engine run time for battery charging on this boat. Reduced engine run time translates directly into a lowered amortized cost of power, and an improved lifestyle. There is another hidden improvement here as well, in as much as the efficiency losses charging and discharging a TPPL battery are only around half of those with conventional lead-acid batteries, so more of the alternator’s output is put to useful work (or, to put it another way, it takes less engine run time at a given alternator output to put back a given amount of energy into the batteries). With lithium, these losses would be virtually non-existent.

Exploiting the potential
Even with the changes we made to Chris’ boat, we are only scratching the surface of the potential inherent in these new battery technologies. Given his nominal 500-amp-hour battery capacity, charging devices of up to 500 amps could be put to use (in fact, at a 50 percent state of charge the TPPL batteries can accept charging currents that are well above 100 percent of the battery’s rated capacity; lithium can easily accept 200 percent of rated capacity). These kinds of charge rates would dramatically reduce the engine run time and the amortized cost of power.

In practice, it is hard to find alternators with outputs greater than 200 amps at 12 volts, and with an output curve that is suitable for battery charging at anchor (it requires the alternator to be wound such that it achieves its rated output at relatively low rpms).

The initial Balmar 97 EHD installation is shown here. Calder discovered he could not maintain enough belt tension on the big alternator and later had to add an idler pulley to increase the belt wrap.

Assuming we could run the alternator at its full rated output (in practice, we’ll never achieve this), at 200 amps and 14 volts the charge rate would be 2.8 kW = 3.6 hp, which, given 50 percent alternator inefficiencies, translates into a load on the engine of 7.2 hp (perhaps a bit higher with belt and other losses). The propulsion engine is still lightly loaded so its operation remains inherently inefficient. At full rated output a 500-amp alternator would get us to 7 kW (9.1 hp), placing a respectable load of close to 20 hp on the engine, but if this is a side load it will void most warranties!

In any alternator installation, so long as there is adequate space, a large frame alternator will invariably have better heat dissipation characteristics than the more typical small frame alternators. Beyond this, there will always be a conflict between providing effective cooling without compromising engine compartment sound shielding.

A much more efficient way to exploit the capabilities of these new batteries is by using a stand-alone DC generator. The electrical end of the generator will be more efficient than an alternator (some have peak efficiencies above 90 percent), and, with proper design, the load can be matched to the engine such that whenever the engine is running, it is always operating at, or close to, its peak efficiency. The net effect is a radical increase in the average charge rate and in the efficiency of the system, with a concomitant reduction in engine run hours and the amortized cost of power.

On my own boat, for the HYMAR experiments we had 12 100-Ah Odyssey batteries wired in series to give us 144v. This equated to a nominal 14.4-kWh battery bank (1,200 Ah at 12v). We had a 22-kW, 144v DC generator from Polar Power. At a 50 percent state of charge, the batteries drove the generator to full continuous output. We could put enough energy into the batteries in 15 minutes to run the boat at anchor for several days!

Deficit reduction
The measures we have taken on Chris’ boat represent an interim approach to reducing the energy deficit on this, and similar, boats. If the TPPL batteries and upgraded alternator and regulator are installed as original equipment, the cost differential over a conventional installation is not that great. If the boat spends any significant time battery charging at anchor, the cost will be quickly paid for in terms of reduced engine run time. Although lithium is considerably more expensive than TPPL on a kWh of capacity basis, if its improved performance can be fully exploited it will prove even more cost effective over the lifetime of the batteries.

A longer-term solution would redesign the DC system to fully exploit the high charge acceptance rates of these new battery technologies. The net result would be a radical reduction in the amortized cost of energy on boats, with greatly reduced engine run times for energy production on off-the-grid boats, and considerably more energy available.

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Contributing editor Nigel Calder is a principal member of the EU Hybrid Marine Project and the author of Boatowner’s Mechanical and Electrical Manual.
 

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