Car Mechanics (UK) - - Vehicle batteries -

As with many con­sum­able com­po­nents, bat­tery spec­i­fi­ca­tion and re­place­ment pro­ce­dures are be­com­ing more be­spoke on newer cars. As a typ­i­cal AGM bat­tery ages, smart alternators ad­just their charg­ing al­go­rithms, which would be in­ap­pro­pri­ate for a brand-new bat­tery. There­fore,

a re­place­ment might need to be regis­tered to the car af­ter fit­ting, but note that this is not nec­es­sary on ev­ery ve­hi­cle.

To find out if it ap­plies to your par­tic­u­lar make and model, en­ter your reg­is­tra­tion num­ber or Ve­hi­cle Iden­ti­fi­ca­tion Num­ber (VIN) at

1 Se­lect a gear or ‘Park’ and en­sure that the hand­brake is en­gaged. Chock the road wheels, if nec­es­sary. Re­search your par­tic­u­lar make and model to see which set­tings might be lost if the bat­tery is dis­con­nected.

2 Switch off all elec­tri­cal loads and re­move the ig­ni­tion key. Some cars lock their doors and ac­ti­vate the alarm when the bat­tery is dis­con­nected, so be wary of the keys be­ing trapped ac­ci­den­tally.

3 AGM bat­ter­ies can be pro­tected from ex­ces­sive en­gine bay tem­per­a­tures with cov­ers. En­sure that you do not cause dam­age as they are re­moved and store them in a safe place. Omit­ting to re­fit the cov­ers will de­crease the new bat­tery’s life.

4 First, dis­con­nect the earth pole. This is the neg­a­tive ter­mi­nal on mod­ern ve­hi­cles. Po­si­tion the ter­mi­nal away from the bat­tery post, so that it does not make ac­ci­den­tal con­tact af­ter be­ing re­moved.

5 Next, dis­con­nect the live (pos­i­tive) con­nec­tion. Note that, in many cases, there may be a pro­tec­tive cover placed over the ter­mi­nal. On this VW, a plas­tic cover hooks over the post be­neath the con­nec­tor.

6 Make a note of the means by which the bat­tery is se­cured to its tray and re­move the clamps. Store the parts care­fully. An in­se­cure bat­tery is not only dan­ger­ous but is also an MOT fail­ure.

7 Lift the bat­tery out of its aper­ture care­fully. In some cases, you may have to dis­mount other com­po­nents to fa­cil­i­tate ac­cess. Do not swing the heavy bat­tery so that it strikes nearby com­po­nents, or body­work.

8 Com­pare the orig­i­nal bat­tery size against your re­place­ment to ver­ify that they are the same. Should the bat­tery be too tall, there is an in­creased risk of the ter­mi­nals strik­ing the bon­net, pos­si­bly lead­ing to a ma­jor short cir­cuit.

9 The re­place­ment bat­tery's volt­age should not be be­low 12.5 volts be­fore you fit it. If found to be so, recharge it with a smart charger and test its volt­age. Check its state of health af­ter a few weeks of use and com­pare it against its spec­i­fi­ca­tions.

10 The new bat­tery should have in­su­la­tor caps over its poles. It has been known for users to for­get to re­move them af­ter the bat­tery has been fit­ted and con­nected! Fit them to the old bat­tery, once they have been re­moved from the car.

11 Many bat­ter­ies will fea­ture a vent plug. No bat­tery should be fit­ted with a vent plug on both sides. Some cars are equipped with a pipe that can be con­nected to the bat­tery – con­sult your car-maker’s in­struc­tions.

12 Check the in­tegrity of both the bat­tery leads and their in­su­la­tion. Some bat­tery ter­mi­nals can crack if over­tight­ened. Should any earth straps be­come ac­ces­si­ble, when the bat­tery is re­moved, check their con­di­tion.

13 Ver­ify that the bat­tery tray is both clean and dry. Con­sider that even a small piece of sharp grit can punc­ture the base of a heavy bat­tery, so en­sure that all for­eign ob­jects have been re­moved.

14 While plas­tic bat­tery trays can split, steel ones rust. Wire-brush away any sur­face rust and treat the sur­face with a rust con­verter, fol­lowed by paint. Al­low any coat­ings to dry be­fore re­fit­ting the bat­tery.

15 As with re­moval, take care not to dam­age any sur­round­ing parts, body­work or ca­bling, when low­er­ing the bat­tery into the tray. Re­fit the fix­ings, as shown in Step 6, but do not over­tighten them.

16 With the bat­tery in place, re­fit any sur­round­ing trim re­moved ear­lier. Con­nect the pos­i­tive ter­mi­nal first, fol­lowed by the neg­a­tive; do not over­tighten their clamp­ing bolts, if fit­ted.

17 Do not for­get to re­fit any cov­ers, es­pe­cially for AGM bat­ter­ies. Many cars have plas­tic guards, to stop the bat­tery from get­ting wet. While wa­ter will not dam­age the plas­tic case, it can cause cor­ro­sion to build on the ter­mi­nals.

18 Af­ter re­con­nec­tion, you might lose some set­tings, such as the clock time, trip com­puter in­for­ma­tion and, pos­si­bly, the ra­dio code. You may also have to reini­tialise cer­tain func­tions, but con­sult your hand­book to be sure.

19 Not all ve­hi­cles that are equipped with emis­sion-sav­ing tech­nolo­gies re­quire new AGM bat­ter­ies to be re­coded. This one does. The op­er­a­tion is per­formed via the universal OBD socket, which is sit­u­ated be­neath the fas­cia on this Volk­swa­gen.

20 Yuasa has its own equip­ment for pro­gram­ming bat­ter­ies, but some more ad­vanced Diy­ori­ented di­ag­nos­tic equip­ment has this fa­cil­ity. Should you need to buy di­ag­nos­tic equip­ment, check if this func­tion is present.

21 Di­ag­nos­tic equip­ment will dis­play the bat­tery man­age­ment func­tion in its main menu. For some cars, you can buy a wire­less OBD in­ter­face and pro­gramme the bat­tery us­ing a mo­bile phone app.

22 On the pic­tured Yuasa pro­fes­sional equip­ment, be­cause it is not ve­hi­cle spe­cific, the car make and model is di­alled in man­u­ally. Al­ter­na­tively, the ve­hi­cle ID can be de­tected au­to­mat­i­cally via the VIN num­ber.

23 You'll need to fol­low the on-screen in­struc­tions. Some de­vices may re­quire you to en­ter the se­rial num­ber printed on the new bat­tery, while other soft­ware may guide you through sev­eral steps, as in­di­cated here.

24 To be regis­tered suc­cess­fully, the new bat­tery must match the spec­i­fi­ca­tions of the orig­i­nal bat­tery. Some cars of­fer bat­tery up­grad­ing op­tions. Should this be the case, the new bat­tery data must be en­tered.

25 Run a global di­ag­nos­tic check be­fore reg­is­ter­ing a new bat­tery and delete any fault codes. This pro­fes­sional con­fig­u­ra­tion tool con­ducts this process au­to­mat­i­cally and warns if any per­ma­nent faults will hin­der the reg­is­tra­tion process.

26 You may have to en­ter the maker of your re­place­ment bat­tery or sim­ply the se­rial num­ber on the bat­tery la­bel. In this case, be­cause a new like-for-like bat­tery is be­ing fit­ted, no pro­gram­ming is re­quired.

27 Af­ter press­ing the ‘En­ter’ key, the bat­tery is regis­tered suc­cess­fully. You can con­duct an­other global di­ag­nos­tic check, prior to dis­con­nect­ing the equip­ment from the car’s OBD socket.

28 Note any ac­ces­sory reini­tial­i­sa­tion pro­ce­dures (such as ra­dio codes, win­dow anti-pinch, et al) by read­ing the ad­vice in the own­ers’ hand­book. Start the en­gine and check that it runs cor­rectly.

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