Sizing a Lead-Acid Battery Bank
(Gel,AGM, or Flooded)
Industry recommendations for house battery bank sizing range from 3x to 4x of anticipated daily needs. This sizing also allows you to replenish the daily needs using a bulk charge in a minimum of time (batteries can be bulk charged until they are 80% full). Typical installations are also sized so that the battery does not discharge below the 50% mark that marine battery manufacturers typically use in their life-cycle estimates (the deeper you discharge a lead acid battery, the shorter its life).
Once batteries are 80% full a typical 3 stage charger will switch to absorption mode, where currents are much lower and charge time per unit energy necessarily longer. Thus, in order to minimize charge time while maintaining acceptable battery cycle life, you want to cycle the battery from 50% or more to 80% full.
- Flooded cells are usually bulk-charged at an ampere rate of about 25% of amp-hour capacity. For example, let's say you have to replace 100 Ah every day and chose to install a 400 amp-hour house battery bank. Using the 25% ratio, the maximum charge acceptance (the rate at which the battery can be safely charged) of your flooded battery during bulk charging would be around 100A. Assuming you have no other charge sources on board, after losses and other loads you would want to consider a 120 ampere (hot) rated alternator.
- If you use Gel cells, you can bulk charge with twice the current that flooded cells can sustain. A 400 amp-hour bank composed of gel cells can safely enjoy the benefits of a 200A charge source and bulk charge in less than 1/2 the time of a flooded cell (due to its higher charge conversion efficiency). Here, multiple small frame or a single large frame alternator may be ideal in order to reduce charge time and engine wear.
- The Lifeline AGM, is not current limited at all during bulk charging according to Concorde, the manufacturer. Theoretically, you could make your bulk charge time as short as you would like. In practice, people are charging AGMs with currents of up to 4x the amp-hour capacity. Accounting for conversion efficiency that would be 18x faster than your typical flooded cell but you'd have to install a charge system capable of delivering 1600 amperes to the battery bank (it would take about 5 minutes per day to replenish the 100 Ah with a 20 kW gen-set). In the real world, two large frame alternators may be more appropriate from a weight/cost standpoint.
Some important points to Remember When charging Batteries:
Any time your charge system is significantly smaller in ampere capacity than the charge acceptance of your battery banks, you need to ensure that the charging system can handle extended periods of maximum output. For example, the alternator that was easily capable of dealing with 400 amp-hours of flooded cell capacity could burn up when you switch to AGMs since AGMs can bulk absorb a lot more charge. However, installing a 4x bigger flooded cell bank could have the same effect.
The charge system has to be properly sized and protected to safely charge your batteries. Assuming you use your engine to charge, a KKK-rated alternator helps, as does thermal management. Alternators should be well ventilated and may even benefit from the Balmar MaxCharge system that senses alternator temperature to avoid costly burn-ups. At the very least use a good 3-step external regulator such as the Next-Step from Ample power. Your batteries and your wallet will thank you.
Lastly, fully recharge your banks from time to time (at least once a month) to ensure that sulphation does not set in. Sulphation can be removed through an equalization process, which is basically a controlled overcharge. Unfortunately, equalization attacks the lead plates and will weaken them over time. Furthermore, it is quite dangerous (the electrolyte will start to boil) and should only be done in controlled conditions and under strict supervision. Thus, equalization should be used as a last resort only.
VRLA batteries like Gel and AGM cannot be equalized using the same settings as wet cells. The electrolyte in them cannot be replenished the way it can in wet cells. Thus, any charger you attach to VRLAs should have the equalization circuit turned off (many chargers have a Gel and AGM setting for just that purpose). Most modern regulators have one way or the other of achieving this, whether it is a set of dip switches, a menu, or adjustment screws.
For the adventerous (or desperate) among us, VRLAs can be equalized under very controlled conditions. I recall instructions on how to do it over at Ample Power but don't seem to be able to find them right now. Since VRLAs are unlikely to need equalization, this step should be a last resort only (prior to recycling the batteries.