DC Charging Philosophy

(Scroll down to see actual charging methodology.
Also below: Idealized Charging Cycle, Number/Size of Batteries Required (AH capacity)

Assuming your boat is large enough (a relative term) to require extended DC capacity, the following discussion should answer most questions. The reasoning for the approach below is to put to use the drive engine for charging ONLY when under load (such as motoring). The battery bank is to hold you over in between. This will be calculated in step one.

You do not want to use your engine for charging at anchor or it will be the most expensive generator you purchase. It's life will be shortened immeasurably and cost much more on maintenance. Extended no load operation of a diesel engine will "glaze" the rings and unfortuantely will cause oil blow by at some point. The exhaust system will also start sooting over. The tell-tale sign of this is that the engine exhaust will start to blacken. This is an often occurance to overland trucks when their drivers pull off to sleep in their cabovers and leave the engine running as a normal practice (to run the A/C or heaters....). We have found that a lot of the cross-country truckers are adding small gensets to their rigs to preclude this from happening. The key point to note is the no-load, extended period engine run time. An occasional or short periord should not be a long term factor.

As we just discussed, if room allows, and more capacity is desired, a dedicated generator is required. An AC genset will provide the needed 120vac power to run appliances and the battery charger druring times the propulsion engine is not running. In order to not need a genset running 24/7, a large enough house battery bank via an inverter will also provide 120vac as needed.

First, you need to calculate your demand for DC power (add up the max DC amps consumed between charge cycles). This amount needs to be doubled because you do not want to go below 50% on a regular basis or the battery life will degrade. When you are calculating, consider worst case, such as anchoring for extended periods. Experience and observation puts this number at 750AH on the low side, 1000AH as average and anything higher as great. If you are in a power vessel instead of sail that has high draw items, this will be even higher. You may be limited by the room on hand. 1000AH takes 4 8D batteries.

Second, now that you have the amount of DC amp hours required, double that number and divide the result by your charging sources capacity (standard alternator, "inverter", generator…) and you will have the approximate time needed to do the charging. With this in mind, it brings up whether your sources are adequate.

This is usually the point, where one decides/figures that one does need that second High Output Alternator (See link on sizing). This is truly the best approach. Keep your standard low amperage alternator to charge your starting battery. Use the second High Output Alternator to charge the house bank. With the correct wiring and switches, there are backups.

Also, this is the point where one may discover that one's desires or needs may be beyond the physical size and limitations produced by their vessel. In this situation, one has to accept these limitations and use the vessel with this in mind. Do not go beyond the limitations or comfort or safety will be compromised. The standard alternator or an upgrade to a ZRD 94amp(12vdc) High Output Alternator is your only smart, safe option. Unlike other vendors, we will let you know if you are close to this situation and recommend not purchasing a renovation and living within the vessel's limitations until you are ready for a vessel upsize that will allow your desires to be installed and met.


Third, power drain. Calculations dictate that 2 HP is required for every 100 DC amps generated. The industry uses an efficiency factor of 50% (Our testing finds that most alternators are more efficient than 50% - closer to 75%, but each should be verified for your use) for motors/generators. This brings this number to 3 HP. A ZRD High Output 220amp Alternator requires apx 6 HP at full output. This should not be a problem for most inboards. We have installed them on 25 hp sail drives. Up to 94 amps requires a single 1/2" belt and over requires two 1/2" belts. Normal installation requires a switch in line to the regulator (another reason for an external regulator) to switch it off for many reasons, but mostly in the event you need the extra power delivered to the propeller - on smaller engines less than 50hp.

Fourth, a battery discussion. If you have wet cells, use them until they die or if you can, sell them. Then, get good (not all brands are the same) AGM batteries. The reason is simple - You would defeat the purpose of high output charging. AGM batteries are designed to take high charging rates and wet cells are not. I would not exceed a charge rate in excess of 100 amps with the typical wet cell battery bank. With AGMs, there is no limit except for the max ampacity for a given cable (normally 2/0 or 4/0) size (wire ampacity).

Lastly, Brackets and Mounting. It is not complicated - It applies to all engine manufactures. This is our specialty. First a bracket holds the alternator and provides for belt tension and adjustment. This assembly is then attached to the engine via angle iron attached to the motor mounts or block. A double groove or serpentine PTO (power take off pulley) is attached to the main crankshaft to turn the second High Output Alternator.

With the basic approach discussed, you should have multiple sources of reliable power. You may even run your Air Conditioning off of the batteries with NO PROBLEM. We hope this has helped those of you that are confused by all the hype out there.

Actual Charging

Referring to the first two items above, charging should be in the 50% to 100% range. When cruising, this range gets a little narrowed down to the 50% to 85% range. This allows for the correct economics and efficiency. After leaving the bulk mode and in the acceptance mode of three stage charging, there comes a point when the charge current is low enough that the payback (time required) is only realistic when on shore power. At any point, when the float stage is reached, charging should be terminated. Do not stay in, or keep float stage on for any length of time. Frankly, it was designed for those that are too complacent to monitor their system and turn it off when charging is completed. Some desire to turn charging on and forget it. Yes, that would be a simple option, but do not expect the batteries to reach their full life expectancy if you elect to perform this as your norm.

For those that have, use, or install equipment (in our opinion, required) like a link 10 or 1000, the battery bank condition is easily monitored. For example, using that four 8D AGM battery bank of 1020 amp hours, one only needs to start charging in the range of minus 500 amp hours to minus 300 amp hours (depending on one's cruising schedule) and terminate charging when around minus 150 amp hours. When one gets back to shore power, the system may then be fully charged to minus 0 amp hours. This assumes a charging efficiency of 100%. One's battery state needs to be monitored. This is done by keeping track of the battery bank's charging efficiency factor (CEF) and load testing when warranted and on a proper schedule. There is no absolute regarding CEF, but as it drops below 80%, your load-testing schedule should be increased. See the data below (chart on right) to see how monitoring the CEF will fit significantly into all of your planning. In fact, if done regularly and correctly, you may eliminate your start battery and will be given months of warning before your battery bank needs to be replaced.

Remember, if you choose to look at the starting battery voltage, it is only an INDICATION of the battery's state of charge if the battery has been at rest for at least 8 hours (standard wet cell lead acid). A battery with an 8-hour rest voltage of 12.4 volts or more will likely start an engine. A battery reporting 11.1 volts after 8 hours will likely NOT START your engine, particularly if you have dirty injectors and the engine does not fire up right away!

Our experience with AGM batteries shows their no load voltage readings after 5-15 miniutes of no load to be reliable. Also, their no load voltage reading is on the order of only 0.05 - 0.20 Vdc higher depending on the load the battery bank was experiencing before the load was removed.

If this methodology/routines are followed "religiously", one will ensure proper charging cycles and that the maximum life of the battery bank will be maintained. Depending on your situation and battery state, there are always minor exceptions. Contact us if you have any questions or non-standard situations that arise.

Idealized Charging Curve
Ideal Charge Curve


The First stage (BULK) of the charge cycle (labeled CHARGE above) supplies full charger output current until the battery reaches the acceptance charging voltage (14.2V typ.). The Second stage (ACCEPTANCE) of the charge cycle continues until the battery is accepting only a small amount of current. The battery is now full. During the final, Third stage (Float) of the charge cycle, the voltage is lowered to maintain the batteries, for long life.

Special Notes:
1.) On the top portion of the combined graphs, the amperage is going from 0 to 100% available output on the verticle axis.
2.) On the bottom portion of the graphs, Equalizaion voltage is typically not applicable to AGM batteries.

House Bank cumulative CEF Readings
House Bank cumulative CEF Readings


The verticle axis is the Charging Efficiency Factor. The horizontal axis shows weeks of charge cycle. The data is for the first 2 years of a 840ah house bank (4 - 4Ds). The first few months of data are not included, but if displayed, it would consist of oscillations between 93% and 99% until an accurate initial stable setting of 99% was reached after the batteries in the battery bank acquired their full potential. This takes aproximately 5-15 cycles of discharge & charge.

A small dip at cycle 5, and the longer one starting around cycle 30 were due periods of battery bank restabilization (regaining full potential) after it was exposed to a short (first dip) then a longer (multiple dips) period of cruising. During these periods, only engine alternator and genset charging were utilized. Because the 50- 85% battery charging rule is normally utilized when cruising, the battery bank is never able to reach a 100% recharged state. When returning to shore power and multiple discharge & charge cycles are completed (similiar to when the battery bank is first installed), the battery bank regains it's full (99%) efficiency rating.


Simplified Description of Alternator Output

During the initial Bulk phase, the voltage regulator senses the battery voltage and sends a processed voltage to the alternator via it's field wire to set the desired initial alternator output voltage. The alternator's initial current output (measured in amps) starts to rise also. Fairly rapidly, it will reach the full rated output for that alternator. Current will be kept at this full output level. The field wire voltage is continually adjusted up to a max point until the battery bank reaches its desired voltage. Alternator output & battery condition are at their desired max voltage. During this entire process, Alternator Current output is being controlled by the battery condition. Initially (assuming a depleted battery bank), there is "little" resistance resulting in full rated alternator current output.

Full alternator current output is maintained until the battery bank reaches the "programed" max voltage. When this point is reached and per Ohm's law (current and resistance are inversely proportional for a given voltage), the current starts to decrease as the resistance starts to rise in the constant voltage charging battery bank. This marks a point of changeover into the second part of the charge cycle - Acceptance phase. During this stage, the alternator output voltage is kept at it's max level while reducing alternator output current as it reacts to the continued rise in resistance within the charging battery bank.

At a programmed, regulator point, the process is completed and the third Float stage is initiated. In this phase, the regulator keeps the field voltage constant to maintain the programmed float voltage. Alternator current output is very low and reacts to any demand or load (resistance) on the system.


Remote Battery Charging

Refer to the link (under Products, Click on Alternators ..., then scroll down to click on Duo Charge under Special Products) on the Balmar Digital Duo Charge to see an example of how to charge a remote battery that is required for items such as a windlass or thruster. The battery may be maintained without requiring a large gauge wire run from the charging source. The remote battery will be maintained just like a start battery. In fact, one may install/use multiple Balmar Duo Chargers to solve a variety of requirements. Direct link to Balmar Duo Charge



Number and/or Size of Batteries required

This is relatively simple and a quick calculation. Of course, the answer may not be the one expected, desired, or possible, ... but here it is. If one has read all of the links in the FAQs concerning sizing of alternators, needs, and above ... this is the net result of applying what was discussed. Think of batteries like a checking/savings account in a bank. Just like one needs to have enough money in their personal accounts to pay for items in between paydays, one needs to have enough amp-hours stored in a battery bank to hold them over in between charge cycles.

Now, the actual Calculation. From earlier discussions, you will have calculated the sum total of the amount of amp-hour consumption you will be using on a daily basis. This number is multiplied by the number of days in between charge cycles. You now have the Total number of AH that will be consumed. To decide on the number, size, ... batteries required in order to accomplish this we use the simple formula {from Real World experience and verifyable (not idealized marketing) data}:
Battery Bank AH Rating Required = (Total AH consumed in between charge cycles) multiplied by 3.57.

For example, if you need/use 150 AH per day (This may be on the low side - Your cruising number may be higher or lower, depending on how you limit your consumption - Refrigeration alone consumes around 50-75 AH per day.) and you want to charge the battery bank every other day (at the earliest), you will be consuming 300 AH in between charge cycles. With this number calculated, multiplying it by 3.57 we now have the sizing of our needed house battery bank - 1,071AH (150AH per day * 2 days * 3.57). This is roughly Four 8Ds. Depending on your vessel, you may not have the room to fit 4 8Ds. With this in mind, one may need to consider lower usage of none required items (lighting), charging more often (sooner), or suplemental in between sources (solar, wind, ocassional dockside shore charging - should be done at least monthly).

With the required Battery Bank AHs calculated, one needs to locate/decide on where to put the batteries and then use the largest allowable battery size (reduces number, wiring, increases efficiency, lowest cost, ...) that will fit in the location(s) selected. For example, needing a battery bank size of 1,000AH, one could use 4 8Ds, 5 4Ds, 7 Group 30HTs, 10 Group 31s or 27s, ...

At this point, you are probably wondering why, where, ... multiplying by 3.57. This is the most overlooked, discounted, misunderstood part of any Batery Bank Sizing calculation. Again, remember this is from Real World experience and verifyable data, not Lab Tests. From your readings above and other ZRD FAQs, recall that the entire battery bank AH rating is not usable, one needs to reduce battery AH ratings by 80% (regardless of manufacturer) to get real world data, and that while one is cruising, normal charging typically uses the 50% to 85% scenario (.35). This works out to be 3.57 (1 / 0.8 * 0.35).