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# Automotive/batteries

Question
Hi Ernie.
if I run 2 , 12 volt batteries in a series, pos to pos neg to neg, and each battery has 700 cranking amps, will I get 1400 amps or will I get 700 continuous (more sustained) amps?
the reason I am asking is, I am going to replace the battery in my genie boom lift and it would be cheaper for me to get 2 smaller batteries.
Thank you
Steve

Kirchoff's Voltage Law (KVL):  Voltages sum around a SERIES circuit LOOP, that is: voltage RISES (SOURCES, such as batteries and generators) summed with voltage DROPS (LOADS, which include resistive items such as lamps and heaters along with resistive/dynamic loads such as motors) will NET to ZERO volts.  To put it another way, the total of the source rises will equal the total of the load drops, balancing the loop.

Kirchoff's Current Law (KCL): Currents will sum at the NODES of a set of PARALLEL branches such that the Current, or sums of Current, ENTERING any node shall be balanced by the Current, or sums of Current, LEAVING that node.

That being the case, a parallel connection of batteries - correctly implemented using the suitable wire gauge or cross-sectional area of bus bar for the anticipated (via calculation) current, with the interfacing surfaces appropriately prepared (clean metal surfaces, greased) and secured using suitably-torqued bolt-and-nut fasteners, with star lock washers for good measure) would enable a summing of each battery's cranking amp capacity, provided that the batteries each have the same open-circuit voltage to begin with, and that each have the same internal and terminal resistance.  Usually, batteries will not be quite equal to each other, and so the energy loading will tend to be higher from the "best" battery(ies) compared with the lesser batteries in the set.

Note that the "pos to pos" and "neg to neg" that you state IS a parallel connection and NOT a series one, and thus would be correct for increased cranking capacity.

(As an aside, some might suggest that "cranking-amp capacity" could be shortened to make another good use for the word "ampacity", a spliced combination word [ amp(ere) + (cap)acity ] which was coined by the electrical industry and is used in both the Canadian (CEC) and US (NEC) electrical codes to rate the maximum "working" amps for conductors so as to keep conductor Temperature rise within the temperature limits for a given insulation so as not to shorten the deployment life of a cable below normal lifetime expectations for the equipment or building in which it is installed.)

Make sure all cables and clamp connectors are in very good to excellent shape - if not, why not use the opportunity to renew them while you are at it - and check and then clean or renew as advisable any or all of the connecting hardware?  On assembly, use any of the automotive or electrical industry terminal dressings which include oils to block water ingress and to otherwise keep air from the terminal interfaces so as to limit corrosion.

Incidentally, the economy of battery choice is weighted by a number of factors, and I am assuming that you are taking these into account.  Usually, higher capacity for a given battery will mean a higher price for that battery, compared with lower-capacity batteries.  At the same time, there is - usually - an "economy of scale" that makes the size-price metric nonlinear, that is, to buy ONE larger unit instead of two (or more) smaller units, sized to collectively equal the capacity of the larger unit, would be the more economical choice.   However, the pricing of batteries (and of most other products) also usually comes down to relative volume of sales for the given sizes, and it may well be that two batteries of a given size that are sold in large volume in a competitive market may well each be priced at less than half the going price for the one battery of same net capacity.

Too, cranking-amp "capacity" is one thing, normally the ability to torque over an engine, to start it under adverse conditions.  Once that engine is running, end of problem.

The other capacity is Amp-hour or A-hr capacity, which usually is used to determine the maximum current you can draw off a deep-cycle (low-current long-time) battery, ie: a 100 A-hr deep-cycle battery in its intended application could be expected to deliver a continuous five amps (5A) for no less that twenty (20) hours ...  20 hours x 5 A = 100 A-hr. That might suggest tha a 10 A draw will be okay for ten hours, or 20A for five hours, however, the A x time product will be progressively less than 100 for increasing current = diminishing returns.

This fact would be important to know IF your boom lift was electric, rather than powered by a prime mover which the battery need only start!  I state this only for completeness!

Sir, I thank YOU for giving me the incentive to put together some more notes on battery applications.  My Americano got cold however, and I am now off to get another one! ;)

Cheers ... EGK
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 Rating(1-10) Knowledgeability = 10 Clarity of Response = 10 Politeness = 10 Comment Wow! My husband and I were very impressed with the speed, extreme in-depth careful answer and personal touch with Ernie!!! Thank you for the educational answer. We would highly recommend Ernie for volunteer of the month. Steve and April Collins

Automotive

Volunteer

#### Ernest (Ernie) Kenward

##### Expertise

The challenges I most enjoy are thoughtful technical questions of a trouble-shooting nature in both electrical, power electronic and mechanical systems, mainly automotive but also machine control and small-machine PLC applications. Please note, however, that I am NOT a walking shop manual! I DO, however, make it a point to have those manuals and other service literature for those vehicles I DO own, and highly recommend that anyone serious about maintenance or modification of their vehicles do the same; MOST of your answers WILL be found there. For that matter, I do NOT go out of my way to acquire shop manuals for any vehicle I do NOT own! That being the case, any general query to me along the lines of "What is the meaning of this code read from the ECU of my 2015 XYZ?" or "Where is the fuse for the windshield washer pump found?" (try your car's electrical distribution panel for a start!) will not go far. What I do offer is a pretty good collection of literature, insights and hands-on experience with 1950s to 1980's Ford products (plus a developing database of information and practice with the Mercedes diesel cars), along with an engineering perspective and the ability to design and implement custom control, electrical and mechanical subsystems for vehicles. For that reason, I am happy to make my thoughts and efforts available to those who are of like mind and/or are seriously making a point of learning about their vehicles. Use the Opportunity to Learn!