If you put all the batteries together, why not use one Tender and connect all the batteries's POSITIVE sides together and negative sides together, the batteries all stay up off one tender !! Do NOT put a positive to a negative terminal, then you start increasing the voltage, Just my $.02
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15. Can I charge more than one battery at a time with a single charger? This is a very general question and its answer will cover many aspects of both battery and battery charger characteristics.
And the answer is both YES and NO, depending on several circumstances. Here are 8 items to consider:
A. Are the batteries connected in series or parallel?
B. Are the batteries the same type, that is, are they flooded, sealed, GEL, AGM, etc.?
C. Are the batteries the same size, that is, do they have the same amp hour capacity?
D. And are the batteries used for deep cycle applications, like large marine batteries that are used to run trolling motors, or are they engine start batteries used for automobiles, motorcycles, sports watercraft, or all terrain vehicles? (This latter type of battery is referred to as SLI, which stands for Starting, Lighting, & Ignition.)
E. Are the batteries discharged to the same level before recharging?
F. What is the nominal output voltage rating of the charger?
G. What is the nominal output current rating of the charger?
H. What type of battery is the charger designed to recharge? What this means is what type of charging algorithm is used? In other words, what voltage levels, current levels, and timing does the charger employ as it recharges the battery?
This seems like an awful lot of questions to ask before we can say “YES” or “NO” to charging more than one battery with a single charger. The short answers are given first, and then a more detailed discussion follows.
SERIES CONNECTIONS: If the answer to question 15.A) is that the batteries are connected in series, where the battery voltages add to make a larger voltage, then for optimum recharging, the answers to questions 15.B), C), D), and E) must be yes. The batteries must be the same type, the same size, used in the same application, and they must be discharged to the same level before they are connected to a battery charger. The answer to question 15.F) is that the nominal output voltage of the charger must equal the total nominal voltage of all of the series connected batteries added together. The answer to question 15.G) is that the nominal output current rating of the charger must match the battery manufacturer’s recommendation
PARALLEL CONNECTIONS: If the answer to question 15.A) is that the batteries are connected in parallel, where the battery voltages must be the same and the battery capacities add, so that the charger behaves as if it is charging a larger battery, then for optimum recharging, the answers to questions 15.B) and D) must be yes. The batteries must be the same type and used in the same application. For question 15.C), although desirable, it is not essential that the batteries be the same size if they are connected in parallel when recharging. Similarly for question 15.E), it is not absolutely necessary that they must be discharged to the same level before they are connected to a battery charger. The answer to question 15.F) is that the nominal output voltage of the charger must equal the nominal voltage of the batteries connected in parallel. Remember, that all of the battery voltages must be the same or they can’t be connected in parallel. The answer to question 15.G) is that the nominal output current rating of the charger must match the battery manufacturer’s recommendation. This gets a little complicated because it affects the maximum size difference, the amp hour capacity difference between the largest and smallest battery connected in parallel. It also affects the limit on the total amp hour capacity of all the batteries connected in parallel.
BATTERY CHARGER DESIGN: SERIES and PARALLEL CONNECTIONS: Whether the batteries are connected in series or parallel, the answer to question 15.H) is that the charger must be designed to provide the output electrical power and timing control for the type of battery being recharged. This includes the output voltage and current discussed in questions 15.F) and G).
DETAILED DISCUSSION to support SERIES and PARALLEL Battery Charging: Let’s start by saying that for the purpose of this discussion, most batteries fall into three categories based on their use or application. These groups are: deep cycle (marine), SLI, or standby power. Within these three application groups we can now consider the type of battery. Flooded and sealed lead acid batteries have different charging requirements. There are also several different types of flooded and sealed batteries. But again, let’s limit the discussion to 3 categories: flooded, sealed GEL, and sealed AGM.
COMPARISION of GENERAL BATTERY CHARGING REQUIREMENTS by TYPE: The maximum recharge voltage is the highest for sealed AGM, and the lowest for sealed GEL, with flooded batteries falling somewhere in between. The exception to this rule is flooded SLI batteries that have antimony added to their lead grids. The highest voltage is delivered during the equalization charge period. Equalization charging will be discussed later. The maximum recharge current is the highest for sealed AGM and sealed GEL, and the lowest for flooded batteries. Most battery manufacturers will specify the maximum recharge current to be a percentage of the amp hour capacity.
For example, many flooded SLI batteries are limited to 10% to 20% of the amp hour capacity. For more specific example, consider a 20 Ah, flooded SLI battery, as you would find in a motorcycle, sports watercraft, or ATV. In this case, the charger should only deliver a maximum charge current of 2 to 4 amps to the battery. On the other hand, sealed AGM batteries are becoming very popular in these SLI applications. Sealed AGM batteries do not usually have the same maximum charge current limitations as flooded batteries. However, some AGM battery manufacturers continue to prefer to make a more conservative recommendation for the maximum charge current.
In this regard, with one more known fact about the majority of commercially available battery chargers, the conservative approach to recommending a maximum charge current is usually not necessary. That fact is that most commercially available battery chargers are not true constant current chargers. What most battery chargers do is one of two things. They either allow the charger output current to immediately taper (reduce in amplitude) in response to an increase in battery voltage, however slight that voltage increase may be, or they maintain a regulated current limit until such time that the battery charger develops sufficient voltage for the charger to switch to a true, constant voltage mode of operation.
The initial period, prior to the constant voltage mode of operation is called the bulk charge period. The constant voltage period is called the absorption charge period. It is during the absorption charge period that the charging requirements for AGM batteries differ most significantly from those for flooded batteries and GEL cells. AGM batteries require a longer period of constant voltage, so long in fact, that the current drawn by AGM batteries is virtually nil for up to several hours at the end of the absorption period. Typically it takes 1 to 2 hours for the battery charge current to drop to a few tenths of an amp at the beginning of the absorption period. After the battery charge current drops to this very low level the AGM battery still requires several more hours with the constant absorption voltage being applied.
The precise electro-chemical requirements for this extended, essentially “zero” current high constant voltage period are debatable. Suffice it to say that a significant body of empirical evidence supports this claim. Without an extended, “zero” current, constant voltage absorption period, the cycle life of AGM batteries is dramatically reduced. The reduction may be by as much as a factor of 2 or 3 to 1. In other words, an AGM battery designed to deliver 400 deep cycles may only deliver 200 or as few as 125 deep cycles if the length of the absorption period is not sufficient. One deep cycle is defined as a battery discharge where the battery capacity is depleted to between zero and 20% of its fully charged value. We could say that the State Of Charge (SOC) of the battery is 0% to 20% after a deep cycle discharge. This is described as a Depth Of Discharge (DOD) between 100% and 80%.
No such lengthy, “zero” current, constant voltage absorption period requirement exits for either flooded SLI or GEL cells. However, both of these battery types do benefit from extended float maintenance charge periods. This is usually referred to a “topping off” the batteries. There is some debate amongst battery and battery charger professionals about the benefits and risks of extended float maintenance charging. The major difference between float maintenance charging and absorption charging is that the float voltage is only a few tenths of a volt above the fully charged, rest state voltage of the battery. This is typically 13.2 to 13.6 volts. This voltage range is below the gassing voltage of the battery. The absorption voltage is about 1 volt higher, 14.2 to 15.0 volts. The absorption voltage range is above the gassing voltage of the battery.
THINGS TO CONSIDER when CHARGING BATTERIES CONNECTED IN SERIES: The highest charge voltage is delivered during the equalization period. For AGM batteries in particular, this higher voltage, between 15.5 and 17 volts for an AGM battery string has an interesting added benefit. For as few as 4 identically sized batteries, discharged to the same DOD, after recharge without an equalization period, the individual battery voltage may vary by as much as 1 volt across the string.
EQUALIZATION VOLTAGE IMPACT on SERIES CHARGING: Let’s consider 4 AGM batteries connected in series. The individual battery voltages would optimally be in the 14.5 to 15.0 volt range, while the charger delivers its absorption voltage. Let’s say 14.7 volts per 12-volt battery for this discussion, or a total of 58.8 volts. Without high voltage equalization, it is possible, in fact even likely after a few discharge / charge cycles, that the lowest battery voltage in the string may be 14.2 volts and the highest battery voltage in the string may be 15.2 volts, with the other 2 battery voltages being 14.9 and 14.5 volts. Within a matter of minutes after the charger applies an equalization voltage of 15.5 volts per 12-volt battery, or 62 volts total, each of the 4 batteries will “snap” in line, varying by no more than 0.2 volts per 12-volt battery.
The optimal timing of this equalization voltage application, much like the extended absorption period, is a subject of debate among industry professionals. Again, the empirical evidence is clear pertaining to the result. If the individual battery voltages in the string are not equally matched, with only a few tenths of a volt per 12 volt battery, then the long term ability of the battery string to deliver a significant percentage of its design deep cycle life is dramatically reduced. An analogy can be drawn to the 6 individual cells comprising a single 12-volt battery. If a single 2-volt cell is weaker than the other 5, then the 12-volt battery will not consistently deliver its design deep cycle capacity over time. This observed result is called “Premature Capacity Loss” or PCL. There are other reasons for a battery to exhibit PCL, but sub-optimal series string charging is certainly one of them.
CHARGING BATTERIES in SERIES that HAVE NOT BEEN DISCHARGED TO THE SAME DOD or BATTERIES that ARE A DIFFERENT SIZE (AH Capacity): Just as a single, weak 2 volt cell will degrade the performance of a 12 volt battery, recharging batteries connected in series that have different beginning SOCs or have been discharged to different DODs will have a similar result. The lowest SOC battery in the string will most likely never recover fully, remaining undercharged, while the highest SOC battery in the string will become overcharged. Either case situation result in premature capacity loss.
Similarly, although at first glance the effect seems to be the opposite, the smallest Ah capacity battery in the string will become overcharged, while the largest Ah capacity battery will likely never be fully recharged. This assumes that that both or all of the batteries have been discharged to the same DOD prior to recharge. Different size, Ah capacity batteries is a more complicated case than same size batteries discharged to different DODs.
An argument could be made that different size batteries could function and be fully recharged, by exerting careful control over both the discharge and the recharge. However, the smaller battery will experience a deeper discharge on each cycle, thereby approaching the end of its cycle life sooner. Over time, the smaller battery will not be able to be fully recovered and it will become the weak “cell” in the single battery analogy.
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If one of the batteries is down to 10V and the other battery is a full 12-13V, the less charged battery will be charged by the other battery when you connect them in parallell. The charge current will only be limited by the resistance in the cables that interconnect the batteries. If you're lucky, all that will happen is that the cables run very hot. If you're unlucky they will melt... If you do this with a pair of starter cables (that are thick enough not to melt), it could actually cause one of the batteries to explode in your face, or if you're lucky, just an acid sprayed car/garage... In any case, one or more of the batteries will not enjoy the treatment.
