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Deep Cycle Battery Selection Confusion
Selecting the correct battery for your electric vehicle conversion can be a confusing process. There are two types of battery categories to select from: lead-acid batteries or lithium based batteries. This post focused on lead-acid batteries, and in particular, deep cycle batteries.
The first thing about deep cycle batteries is that there is a difference between the typical starter battery, that you can find in your local auto parts retailer and a deep cycle battery. The difficult thing part is identifying them apart as they both look the same on the outside. When looking at the materials used internally it is true that they both are lead-acid batteries, that is there are two lead plates flooded in an acid electrolyte. To be a bit more precise the positive plate is a lead peroxide, the negative plate is a pure lead in soft sponge condition. The electrolyte is a dilute sulphuric acid which floods the battery cell and enables the chemical reaction to produce the electric charge.
Starter battery versus flooded deep cycle
With the basic operation and makeup of a lead acid battery identified there still remains the question of the difference between the starter battery in my car and a deep cycle battery. The first thing you will find is a price difference, and more importantly a weight difference. The flooded lead-acid deep cycle battery will always weigh more than a similar sized starter battery. Though both lead-acid batteries use the same materials in the plates and electrolyte, the desired operation of the two types of batteries is what sets them apart.
Starter batteries used with internal combustion engines are intended to crank over the engine at a high torque, low speed, condition. This is required to kick start the combustion cycle as the compression forces need to be built up and fire through a couple of full power cycles. Starter batteries are required to power the starter motor with a high current during cranking until the power stroke is strong enough to sustain the rotation of the crankshaft and the reciprocal pistons. Starter batteries are designed to provide the starter motor with a high current in order to get the engine running. After the engine has been idling, the battery is used for its secondary purpose, which is to provide power for accessories.
This is not its strong suit as anyone that has gone for a double header drive-in movie can attest to. For those who do not know what I am talking about, there used to be a lot of drive-in movie theaters, which required a movie goer to keep the battery on throughout the entire movie. After two hours of use the starter battery showed signs of trouble when using accessories like speakers for an extended period of time. The common solution to this particular problem is to start the engine for 15 minutes, during intermission, so that the alternator can recharge the starter motor. This would allow the viewer to watch the next movie and be able to drive home after.
Working principle of lead-acid battery
Design of Deep Cycle Battery
3 Types of Deep Cycle Lead Acid Batteries
1. Flooded lead-acid battery
The flooded lead-acid battery deep cycle battery, as stated above, is similar to the flooded lead-acid starter battery. Straight forward change with thicker positive and negative plates, and historically the most common available battery in automotive parts retail stores. Since the lead-acid is free to flow between plates, the battery must be mounted horizontally and cannot be on its side due to leakage issues. They are available in either 6-volt, typically labeled as marine, and 12-volt battery configurations. In the event of an overcharge, the flooded electrolyte will release water into the atmosphere affecting the performance of the battery and amount of life remaining in the battery.
2 – Absorbed Glass Mat (AGM) battery
The absorbed glass mat, or AGM battery, is relatively new to the renewable energy industry. The plates are not initially solid in this type of battery as they are in the flooded lead-acid battery plates. Both the positive and negative plates are made with grids which are where the electrolyte paste is stored. The absorbent glass mat is wrapped around each positive plate to protect against shorts and allow the chemical reaction to take place between the positive and negative plates. The battery is made by stacking the negative and positive plates until the desired size or capacity is met. This denser design utilizes the internal space in the battery by reducing unused space in the battery used for the liquid lead-acid in the flooded battery.
The AGM battery is sealed, maintenance free, non-spillable, and considered a dry cell deep cycle battery. AGM batteries can be mounted in any orientation. A specific AGM charger is required to control the charging and to prevent overcharging. In the event of an overcharge, the safety regulated valve is opened temporarily to discharge the gasses.
3 – Gel battery
Gel batteries are similar to flooded lead-acid batteries with the electrolyte being gel instead of liquid. With the gelled electrolyte, the battery can be mounted in any position. Gel batteries are sealed and do not require any maintenance during its lifespan. Since the battery is sealed there are no gasses released into the environment. There is also a microporous separator between the positive and negative plates.
Gel batteries are sealed, maintenance free, non-spillable, and can be mounted in any orientation. A specific gel charger is required to control the charging to a very precise voltage and to prevent overcharging. In the event of an overcharge, the safety regulated valve is opened temporarily to discharge the gasses.
With the different types of deep cycle batteries addressed, we can look at the electrical specifications that are listed on each battery. Once you start looking into each battery you will notice technical specifications such as BCI group number, MCA or CCA, Reserve Minutes at a set amperage, and number hour rate amp/hour rate number.
BCI Group Number
The battery council international, or BCI, has standardized battery sizes into group numbers or types, the BCI group numbers determine the physical size of the battery. The group number does not have a direct link to the battery amp hours rating. As the physical size of the battery increases, the amp hours rating will increase as well.
Battery Stuff has put together a good list of common battery case sizes by BCI group number with the battery types, examples of popular brands, the dimensions, and amp hours. Battery Web has a longer list of BCI group numbers, dimensions, polarity, and terminals.
Both cranking amps and cold cranking amps are tested with the same method, just at different temperatures. Cranking amps, or CA, are measured at 32°F (0°C), while cold cranking amps, or CCA, are measured at 0°F (-17°C). There are also marine cranking amps, or MCA, which are the same as cranking amps.
The cranking amps are more important for deep cycle batteries. Typically the cranking amperage is 80% of the listed cold cranking amperage at 0°F (-17°C). The main takeaway is that a battery will discharge current at different amounts when at different temperatures, and battery current is heavily dependent on the temperature. There may also be hot cranking amps rating, or HCA, which will be the highest number since it is measured at 80°F (26°C). The HCA rating is not approved by the battery council international, only the CA, and CCA ratings are approved.
Batteries Northwest provides a breakdown on cranking amps in their battery school.
Battery University provides more information on how to measure cold cranking amps.
Capacity Amp Hours
When looking at all the technical specifications the amp-hours, or AH, capacity is the most important for deep cycle batteries. The amp-hour discharge rate is driven by C-rates. C-rates are a standard time-based identifier to indicate the discharge rate with a time element involved. The C-rating scale is based on the relation to 1C which is 1 hour. An example would be 50 amp-hours at 1C would mean that 50 amps are discharged over 1 hour. Using the same battery, now with a 5C discharge rate, could be 35 amp-hours, which means that 35 amps are discharged over 12 minutes, or 5 times faster than 1C within 1 hour.
On the other side of the C-rating, lower than 1C numbers, is where most deep-cycle batteries are rated. Typically you will see a 20-hour rating, which is the same as 1/20 C, or a 0.05C discharge rating, which is the indicated discharge rate the battery will be fully discharged. For a deep cycle flooded lead-acid battery, here is an example from Trojan Battery Company for model J185E-AC.
The 20-hour rate is 175 amp-hours capacity, which means that to discharge this battery over 20 hours the discharge current will have to be 175 capacity amp-hours divided by 20 hours, or 8.75 amp-hours. To bring some clarity to this subject Trojan Battery Company provides the capacity in minutes with respect to the current. When looking at this battery the 5-hour rate of 144 capacity amp-hours is 28.8 amp-hours over 5 hours, which is close to the indicated 312 minute capacity with constant discharge at 25 amps.
Please refer to this link from Optima Batteries for an AGM battery example, look at model Bluetop 34M.
Battery University describes what is C-rate in their course BU-402.
When looking into which battery is right for your electric vehicle conversion, the capacity amp-hours will eventually determine the vehicle’s range. The battery does not control the discharge rate, but rather the electric motor and other components. When selecting batteries you can estimate the discharge C-rate, and this will determine the capacity runtime that your battery system will be able to discharge based on the demand.
Here is a list of the deep-cycle reliant AGM batteries from Trojan Battery Company. As you will notice, the batteries with the highest capacity amp-hours are the heaviest 12-volt batteries with the largest dimensions. If you are tempted by the 6-volt batteries just remember that you will need to have twice as many batteries as the 12-volt battery to get to your desired system voltage.