How to Charge a Deep Cycle Battery Properly
Deep cycle batteries may be recharged in many ways, but the best thing to do is to follow recommendations by their manufacturers regarding charging voltages, temperatures, and currents.
However, in real-life situations, such ideal charging conditions are not always possible.
Updated: September 23, 2022.
Features and Specifications of Deep Cycle Batteries
Deep cycle batteries are optimized for deep discharge applications and are cycled a lot. Large deep cycle batteries with capacities of 100-200 or even more Ah may be even used as dual-purpose batteries and can crank smaller gas and even diesel engines, but these batteries are not intended for such applications.
Deep cycle batteries differ in:
- size/capacity: small deep cycle batteries are commonly found in Uninterruptible Supply Systems (UPS), medical and security equipment, and they are commonly used as, for example, fish finder batteries and similar. Larger deep cycle batteries are used in off-the-grid applications, as solar batteries, as backup batteries for mission-critical applications, as RV house batteries, etc.
Since the capacity of deep cycle batteries varies from just a few Ah to hundreds of Ah, so does vary the current (given in Amps) required to recharge the battery in the required/acceptable time properly.
- voltage: deep cycle batteries are mostly 12V units, but there are also 6V, 24V, 36V, etc. batteries.
Of course, these are just 'nominal' voltages, with the actual voltages ranging from 10 or even fewer volts to almost 15 volts for a single "12V" battery.
Note: when the lead-acid batteries are charged to 15 or even more volts, they will start to release hydrogen and oxygen - such battery overcharging can significantly shorten the battery's life and create potentially very hazardous situations since hydrogen is a very flammable gas.
- chemistry: deep cycle batteries are mostly lead-acid and lithium batteries.
Lead-acid batteries are Wet/Flooded, Enhanced Wet/Flooded, Absorbent Glass Mat (AGM), and Gel-Cell batteries.
Flooded lead-acid batteries are rarely used due to the liquid electrolytes, increased corrosion of the battery terminals, and the constant requirement of monitoring the electrolyte levels.
AGM and Gel-Cell (gel batteries) lead-acid batteries are non-spill lead-acid batteries that can operate in any position, except upside down, offering many benefits over flooded lead-acid batteries.
Each lead-acid and lithium battery has a nominal voltage of ~12 volts (for a six-cell battery lead-acid battery and a four-cell Lithium Iron Phosphate battery), but the actual charging voltages differ slightly.
Lithium batteries differ in chemistry significantly, but the most popular is the Lithium Iron Phosphate (LiFePO4) chemistry, which is also the safest lithium battery chemistry.
To increase the safety even further, both starting and deep cycle Lithium Iron Phosphate (LiFePO4) batteries come with built-in Battery Management Systems which protect the battery from unwanted events like over-discharge, over-charge, over-temperature, short-circuit, and similar.
- end voltage of the charged deep cycle battery also depends on the battery's temperature - each type of lead-acid battery has slightly different temperature coefficients, which must be used to calculate the optimal end charging voltage.
The charging end voltage of the lithium batteries is often determined automatically via the built-in BMS - when the end charging voltage is reached, the BMS will cut off the battery from the charger, preventing overcharging.
However, good lithium chargers should not charge lithium battery to their cutoff voltage - if that start to happen regularly, then there is some issue with the battery and/or its charger.
Deep Cycle Battery Charging Methods
When the deep cycle battery is connected to the electric system of some device, vehicle, boat, or similar, let the built-in alternator/charger recharge the battery as it is intended to do.
However, when recharging deep cycle batteries in other situations, there are several methods how to charge them ... and not all of them are the proper way to do it, but if required, such methods may help ...
Battery Chargers
Modern lead-acid battery chargers are smart/intelligent battery chargers that analyze the battery first and then charge the battery according to the battery condition.
A good lead-acid battery charger should have the option of setting the exact lead-acid battery type (wet, AGM, Gel-Cell, calcium, etc.), battery use (cycle use, float use), and automatic temperature compensation, preferably with the temperature probe placed very near the battery.
Maximum charger current should allow the battery charger to recharge the battery in 5-10 hours fully - the actual charging process usually lasts a little bit more since the battery charger doesn't charge the battery all the time with the maximum charging current.
And when the battery is fully recharged, a good battery charger should automatically switch into maintenance mode and keep the battery fully charged indefinitely.
For example, very popular 12V 75-100 Ah batteries may be fully recharged using intelligent 10-15 Amps AGM battery chargers rather quickly, without over-stressing the batteries.
It is always recommended to check the battery's Owners Guide and to find out the 'ideal' and the maximum charging current and end voltages.
Lithium batteries should only be recharged with the dedicated lithium battery chargers or with the smart AGM battery chargers that feature a dedicated lithium battery charging mode.
The charging algorithm of lithium batteries differs from the charging algorithm of lead-acid batteries. Lithium batteries are commonly charged using the CC/CV charging algorithm - first, the battery is charged using Constant Current, and after the voltage reaches a certain level, the battery charger switches to the Constant Voltage mode, which ends when the current drops below a certain level (depends on the battery capacity, for example).
If the lithium battery is being charged with the smart AGM battery charger using lead-acid battery charging mode (wet/flooded, AGM, Gel-Cell), BMS may be damaged, or at least it can cut off the battery when the smart lead-acid battery starts to probe the battery or even start to 'desulfate' it.
And if the charging process continues, lead-acid batteries are generally charged to somewhat lower values than lithium batteries which then cannot be fully charged using smart lead-acid battery chargers.
For short, charge the lithium batteries using lithium battery chargers or smart AGM battery chargers with lithium battery charging modes.
Lithium batteries accept charge very well, and their stronger charging currents allow them to be recharged faster, for example, 50-100 Ah lithium batteries may be recharged using 20 Amps battery chargers.
Of course, before charging the lithium battery, check its maximum charging current, just in case.
Charging the deep cycle batteries with the battery chargers is the proper way of charging such batteries unless they are connected to the electric system of a car, truck, RV, boat, etc.
But, in real-life situations, deep cycle batteries may be recharged in many ways, and some of them are not 'ideal,' but they may help.
Car Alternator
The car alternator is used to keep the car's built-in battery charged and to power various loads while the engine is turned On.
Also, a car alternator may be used for charging other batteries, not just the built-in battery.
The principle is simple - turn on the engine, and using jump cables, connect the car's battery (positive terminals first) with the empty, deep cycle battery and let it charge for some time.
This method is simple and easy to do, especially when being off-the-grid and far away from any mains power outlets.
However, there are several issues with this method, including:
- absolutely no control over the charging current,
- smaller and mid-range deep cycle batteries should not cause issues with the line voltage, but larger batteries may pull so much current that the car's voltage drops below a safe level, causing the car's engine to shut off - in the best case. In the worse case, the alternator can get damaged. In the worst case, the car can end up in flames.
So, if You have a car with a 60-70 Ah battery, feel free to recharge a similar battery for 5-10 minutes, but not more - even that can be an issue! If possible, check the car's manual or at least monitor the alternator's temperature.
If You want to charge the deep cycle battery using your alternator, it is highly recommended to connect the battery to the alternator via DC to DC battery charger. Such battery chargers are usually connected via D+ wire to the ignition, and they don't operate unless the engine is turned On, preventing the main starting battery from being discharged completely.
When dimensioning DC to DC battery chargers, it is very important to know the Amps rating of your alternator. For example:
- If the current of your alternator is <160 Amps, a 20 Amps DC battery charger is recommended,
- If the current of your alternator is around 160-180 Amps, a 40 Amps DC battery charger is recommended,
- If the current of your alternator is 180 Amps, preferably 200+ Amps, a 60 Amps DC battery charger is recommended.
Note: one of the bigger issues when charging the deep cycle battery is alternator cooling while the engine is idling or at low RPM since the alternator cooling is limited and DC to DC charger is pulling additional Amps. If You notice any problem with the alternator while idling (overheating, for example), turn off the DC to DC charger and check how the alternator behaves.
Should You decide to recharge the deep cycle (or any other battery) using a car alternator, whatever happens, it is your own responsibility.
Car's Cigarette Lighter Adapter
Car's cigarette lighter adapter features 10 Amps circuit breaker, rarely more.
A car's cigarette lighter adapter can be used to charge batteries directly, but the user has no control over the charging current - at least when the battery is connected directly.
It is highly recommended to have the engine turned ON when the cigarette lighter adapter is used for charging the batteries or for powering other similar loads - car inverters and similar.
Note: certain cars come with cigarette lighter adapters with limited operating time, especially when pushed to the maximum. If they overheat, they shut themselves off automatically ... and that is the best-case scenario.
Power Generators
Power generators convert the chemical energy of the fuel into electric energy.
Many smaller power generators come with 12V 8-10 Amps charging ports, allowing the user to directly charge lead-acid batteries that don't require stronger charging currents.
However, larger lead-acid and all lithium deep cycle batteries should be recharged using smart battery chargers powered by the generators directly - using their 120/240V outputs.
Such configuration can recharge deep cycle batteries when off-the-grid, when camping, in emergencies, and similar.
For example, a 2000W inverter power generator can easily power 3 (three) 12V 15 Amps battery chargers - that three battery chargers with an efficiency of, let's say, 80%, and other losses don't require more than 700-800 Watts of power.
Solar Charge Controller
Solar MPPT or PWM charge controller monitors the power provided by the connected solar panel (or panels) and charges the battery in the most efficient way possible.
Charging current depends on the light conditions, number of solar panels, battery size, capacity, chemistry, discharge level, etc.
Charging the deep cycle battery with the solar charge controller is one of the most popular methods for people used to camping, hiking, fishing, or living off-the-grid in general.
Long Story Short: Deep cycle batteries may be recharged in many ways, but if You have a discharged deep cycle battery on your workbench, the best course of action is to recharge it using a smart battery charger with the charging mode intended for the battery that you have.
Other charging methods are also plausible, especially charging the battery using a solar charge controller (with solar panels) and the power generator.
If You intend to recharge your deep cycle battery using the car's alternator, please, be very careful and, if possible, check the car's documentation. Do NOT overheat your alternator or any other component/wire since they can easily burst into flames ...