Chuong 2
36Chng 2: Accu khi ong35He thong ien va ien t tren oto hien ai
he thong ien ong c
2.1 Nhiem vu va phan loai accu oto
a. Nhiem vu
Accu trong o to thng c goi la accu khi ong e phan biet vi loai
accu s dung cac lanh vc khac. Accu khi ong trong he thong ien thc
hien chc nang cua mot thiet b chuyen oi hoa nang thanh ien nang va
ngc lai. a so accu khi ong la loai accu ch axit. ac iem cua loai
accu neu tren la co the tao ra dong ien co cng o ln, trong khoang
thi gian ngan (5(10s), co kha nang cung cap dong ien ln (200(800A)
ma o sut the ben trong nho, thch hp e cung cap ien cho may khi ong
e khi ong ong c.
Accu khi ong con cung cap ien cho cac tai ien quan trong khac
trong he thong ien, cung cap tng phan hoac toan bo trong trng hp
ong c cha lam viec hoac a lam viec ma may phat ien cha phat u cong
suat (ong c ang lam viec che o so vong quay thap): cung cap ien cho
en au (parking lights), radio cassette, CD, cac bo nh (ong ho, hop
ieu khien), he thong bao ong
Ngoai ra, accu con ong vai tro bo loc va on nh ien the trong he
thong ien o to khi ien ap may phat dao ong.
ien ap cung cap cua accu la 6V, 12V hoac 24V. ien ap accu thng
la 12V oi vi xe du lch hoac 24V cho xe tai. Muon ien ap cao hn ta
au noi tiep cac accu 12V lai vi nhau.
b. Phan loai
Tren oto co the s dung hai loai accu e khi ong: accu axit va
accu kiem. Nhng thong dung nhat t trc en nay van la accu axit, v so
vi accu kiem no co sc ien ong cua moi cap ban cc cao hn, co ien tr
trong nho va am bao che o khi ong tot, mac du accu kiem cung co kha
nhieu u iem.
2.2 Cau tao va qua trnh ien hoa cua accu ch-axit
2.2.1 Cau tao
Accu axit bao gom vo bnh, co cac ngan rieng, thng la ba ngan
hoac 6 ngan tuy theo loai accu 6V hay 12V.
Hnh 2.1: Cau tao bnh accu axit
Trong moi ngan at khoi ban cc co hai loai ban cc: ban dng va ban
am. Cac tam ban cc c ghep song song va xen ke nhau, ngan cach vi
nhau bang cac tam ngan. Moi ngan nh vay c coi la mot accu n. Cac
accu n c noi vi nhau bang cac cau noi va tao thanh bnh accu. Ngan
au va ngan cuoi co hai au t do goi la cac au cc cua accu. Dung dch
ien phan trong accu la axit sunfuric, c cha trong tng ngan theo mc
qui nh thng khong ngap cac ban cc qua 10 ( 15 mm.
Vo accu c che tao bang cac loai nha ebonit hoac cao su cng, co o
ben va kha nang chu c axit cao. Ben trong vo c ngan thanh cac
khoang rieng biet, ay co song khoi ban cc tao thanh khoang trong
(gia ay bnh va khoi ban cc) nham chong viec chap mach do chat tac
dung ri xuong ay trong qua trnh s dung.
Khung cua cac tam ban cc c che tao bang hp kim ch stibi (Sb) vi
thanh phan 87 ( 95% Pb + 5 (13% Sb. Cac li cua ban cc dng c che tao
t hp kim Pb-Sb co pha them 1,3%Sb + 0,2% Kali va c phu bi lp bot
dioxit ch Pb02 dang xop tao thanh ban cc dng. Cac li cua ban cc am
co pha 0,2% Ca + 0,1% Cu va c phu bi bot ch. Tam ngan gia hai ban
cc lam bang nha PVC va si thuy tinh co tac dung chong chap mach gia
cac ban cc dng va am, nhng cho axit i qua c.
Hnh 2.2 : Cau tao khoi ban cc
Dung dch ien phan la dung dch axid sulfuric H2SO4 co nong o 1,22
( 1,27 g/cm3, hoac 1,29 (1,31g/cm3 neu vung kh hau lanh . Nong o
dung dch qua cao se lam hong nhanh cac tam ngan, rung ban cc, cac
ban cc de b sunfat hoa, khien tuoi tho cua accu giam. Nong o qua
thap lam ien the accu giam.
Hnh 2.3: Cau tao chi tiet ban cc1. Ban cc am; 2. Ban cc dng; 3.
Vau cc; 4. Khoi ban cc am; 5. Khoi ban cc dng.
2.2.2 Cac qua trnh ien hoa trong accu
Trong accu thng xay ra hai qua trnh hoa hoc thuan nghch ac trng
la qua trnh nap va phong ien, va c the hien di dang phng trnh
sau:
PbO2 + Pb + 2H2SO4 ( 2PbSO4 + 2H2O
Trong qua trnh phong ien, hai ban cc t PbO2 va Pb bien thanh
PbSO4. Nh vay khi phong ien, axit sunfuric b hap thu e tao thanh
sunfat ch, con nc c tao ra, do o, nong o dung dch H2SO4 giam.
Qua trnh phong ien Ban cc amDung dch ien phanBan cc dng
Chat ban au Pb2H2SO4 + 2H2OPbO2
Qua trnh ion hoaSO4- -, SO4- -,4H+ 4OH - Pb++++
Qua trnh tao dongPb++ - 2 e- Pb+++2e-
Chat c tao ra PbSO44H2O-2H2O
2H2O PbSO4
Qua trnh nap ienBan cc amDung dch ien phanBan cc dng
Chat c tao ra cuoi qua trnh phong PbSO4 4H2O PbSO4
Qua trnh ion hoa Pb++, SO4- -2H+, 4OH -, 2H+ SO4- -, Pb++
Qua trnh tao dong+ Pb++++
Chat ban au Pb2H2OH2SO4 H2SO4PbO2
S thay oi nong o dung dch ien phan trong qua trnh phong va nap
la mot trong nhng dau hieu e xac nh mc phong ien cua accu trong s
dung.
2.3 Thong so va cac ac tnh cua accu ch-axit
2.3.1 Thong so
a. Sc ien ong cua accuSc ien ong cua accu phu thuoc chu yeu vao
s chenh lech ien the gia hai tam ban cc khi khong co dong ien
ngoai.
- Sc ien ong trong mot ngan
ea = (+ - (- (V)
- Neu accu co n ngan Ea = n.ea.
Sc ien ong con phu thuoc vao nong o dung dch, trong thc te co
the xac nh theo cong thc thc nghiem:
Eo = 0,85 + (25oC
(2.1)
Eo : sc ien ong tnh cua accu n (tnh bang volt).
(: nong o cua dung dch ien phan c tnh bang (g/cm3) quy ve +
25oC.
(25oC = (o 0,0007(25 t)
t : nhiet o dung dch luc o.
(o : nong o dung dch luc o.
b. Hieu ien the cua accu
- Khi phong ienUp = Ea - Ra.Ip
(2.2)
- Khi nap ien
Un = Ea + Ra.In
(2.3)
Trong o:Ip - cng o dong ien phong.
In - cng o dong ien nap.
Ra - ien tr trong cua accu.
c. ien tr trong accuRaq = Rien cc + Rban cc + Rtam ngan + Rdung
dchien tr trong accu phu thuoc chu yeu vao ien tr cua ien cc va
dung dch. Pb va PbO2 eu co o dan ien tot hn PbSO4 . Khi nong o dung
dch ien phan tang, s co mat cua cac ion H+ va SO42- cung lam giam
ien tr dung dch. V vay ien tr trong cua accu tang khi b phong ien
va giam khi nap. ien tr trong cua accu cung phu thuoc vao nhiet o
moi trng. Khi nhiet o thap, cac ion se dch chuyen cham trong dung
dch nen ien tr tang.
d. o phong ien cua accu
e anh gia tnh trang cua accu, ta s dung thong so o phong ien. o
phong ien cua accu tnh bang % va c xac nh bi cong thc:
(2.4)
(n - (p = 0,16 g/cm3Trong o: (n - nong o dung dch luc nap
no.
( - nong o dung dch luc o a qui ve 25oC.
(p nong o dung dch luc accu a phong het.
e. Nang lng accu
Nang lng cua accu luc phong ien:
Wp = 3600. Qp. Up(J)
(2.5)
Wp = 3600
n - so lan o.
Nang lng cua accu luc nap ien:Wn = 3600
(2.6)
Trong o:Qp - nang lng phong cua accu.
Up - ien the phong cua accu.
tn - thi gian nap accu.
f. Cong suat cua accu
Pa = IE = I(IR + IRa)
(2.7)
R - ien tr tai ben ngoai.
Pa = I2R + I2RaCong suat a ra mach ngoai (a vao tai ien)
Pa = IE - I2Ra
= E - 2RaI at cc ai khi bang khong ( I =
(2.8)
Nh vay, khi R = Ra , accu se cho cong suat ln nhat.
2.3.2 ac tnh
a. ac tuyen phong nap cua accu
ac tuyen phong cua accu n: khi phong ien bang dong ien khong oi
th nong o dung dch giam tuyen tnh (theo ng thang). Nong o axit
sulfuric phu thuoc vao lng axit tieu ton trong thi gian phong va tr
lng dung dch trong bnh.
a. Thi gian phong S o phong va ac tuyen phongb. Thi gian nap S o
nap va ac tuyen nap
Hnh 2.4: ac tuyen phong - nap cua accu axitTren o th co s chenh
lech gia Ea va Eo trong qua trnh phong ien la v nong o dung dch cha
trong chat tac dung cua ban cc b giam do toc o khuech tan dung dch
en cac ban cc cham, khien nong o dung dch thc te trong long ban cc
luon luon thap hn nong o dung dch trong tng ngan.
Hieu ien the Up cung thay oi trong qua trnh phong. thi iem bat
au phong ien, Up giam nhanh va sau o giam ty le vi sc giam nong o
dung dch. Khi trang thai can bang th Up gan nh on nh. cuoi qua trnh
phong (vung gan iem A) sunfat ch c tao thanh trong cac ban cc se
lam giam tiet dien cua cac lo tham dung dch va lam can tr qua trnh
khuech tan, khien cho trang thai can bang b pha huy. Ket qua la
nong o dung dch cha trong ban cc, sc ien ong Ea va hieu ien the Up
giam nhanh va co chieu hng giam en khong. Hieu ieu the tai iem A c
goi la ien the cuoi cung.
Khi nap ien, trong long cac ban cc axit sunfuric tai sinh. Nong
o cua dung dch cha trong cac ban cc tr nen am ac hn, do o Ea khi
nap ln hn Eo mot lng bang (E, con hieu ien the khi nap: Un = Ea +
In.Ra. cuoi qua trnh nap sc ien ong va hieu ien the tang len kha
nhanh do cac ion H+ va O2- bam cac ban cc se gay ra s chenh lech
ien the va hieu ien the accu tang vot en gia tr 2,7V. o la dau hieu
cua cuoi qua trnh nap. Khi qua trnh nap ket thuc va cac chat tac
dung cac ban cc tr lai trang thai ban au th dong ien In tr nen tha.
No ch ien phan nc tao thanh oxy va hydro va thoat ra di dang bot
kh.
b. Dung lng cua accuLng ien nang ma accu cung cap cho phu tai
trong gii han phong ien cho phep c goi la dung lng cua accu.
Q = Ip.tp(A.h)
(2.9)
Hnh 2.5: S phu thuoc cua dung lng accu vao dong phong
Nh vay dung lng cua accu la ai lng bien oi phu thuoc vao che o
phong ien. Ngi ta con a ra khai niem dung lng nh mc cua accu Q5,
Q10, Q20 mang tnh quy c ng vi mot che o phong ien nhat nh nh che o
5 gi, 10 gi, 20 gi phong ien nhiet o +30oC. Dung lng cua accu c ac
trng cho phan gach cheo (hnh 2.4). Che o phong ay la che o nh mc
nen dung lung nay chnh bang dung lng nh mc cua accu.
Qm = Q = 5,4A.10h = 54Ah
Tren o th (hnh 2.6) bieu dien s thay oi ien the accu theo thi
gian phong trong trng hp accu phong vi dong ien ln I = 3Qm (Che o
khi ong) nhiet o +25oC va - 18oC.
Cac yeu to anh hng ti dung lng cua accu:
Khoi lng va dien tch chat tac dung tren ban cc.
Dung dch ien phan.
Dong ien phong.
Nhiet o moi trng.
Thi gian s dung.
Dung lng cua accu phu thuoc ln vao dong phong. Phong dong cang
ln th dung lng cang giam, tuan theo nh luat Peukert.
. tp = const
(2.10)
Trong o: n la hang so tuy thuoc vao loai accu (n = 1,4 oi vi
accu ch)
Tren hnh 2-5 trnh bay s phu thuoc cua dung lng accu vao cng o
phong. T hnh 2-6 ta co the thay khi accu phong ien nhiet o thap th
ien dung cua no giam nhanh. Khi nhiet o tang th ien dung cung tang.
Nhng khi nhiet o cua dung dch ien phan cao qua (ln hn +45oC) th cac
tam ngan va ban cc rat mau hong, lam cho tuoi tho cua accu giam i
nhieu.
Hnh 2.6: ac tuyen phong cua accu axit nhng nhiet o khac nhau
c. ac tuyen volt-ampereac tuyen VOLT-AMPERE cua accu la moi quan
he gia hieu ien the cua accu va cng o dong ien phong nhiet o khac
nhau.
Hnh 2.7: ac tuyen Volt Ampere cua accu
Phng trnh mo ta ac tuyen Volt Ampere cua accu: Ua = Ub IpRaTrong
o: Ub - ban au xac nh theo cong thc thc nghiem.
Inm - dong ngan mach luc Ua = 0.
Ub - InmRa = 0
Inm = Ub/Ra
(2.11)
Ub = n(2,02 + 0,00136t 0,001(Qp).
Inm = n+ I+.
I+ = 2,24 + 1,75t 0,4(Qp
(2.12)
n: so ngan accu.
t: nhiet o cua dung dch ien phan (0C).
(Qp: o phong ien accu (%Qp).
n+: so ban cc (+) c ghep song song trong mot ngan.
I+: cng o dong ien i qua mot ban cc dng luc ngan mach.
T ac tuyen Volt Ampere ta co the xac nh ien tr trong cua
accu:
Ra =
d. ac tuyen lam viec cua accu tren otoAccu lam viec tren oto
theo che o phong nap luan phien tuy theo tai cua he thong ien. ien
the nap on nh nh co bo tiet che.
Umf = 13,8 en 14,2V
In = (Umf - Ua() / (R(
(2.13)
(R = Ra + Rdd + RmfTrong o: Rdd : ien tr day dan.
Rmf : ien tr cac cuon stator may phat.
Hnh 2.8: Che o phong nap cua accu tren xe
e anh gia mc can bang nang lng tren xe, ngi ta xem xet he so can
bang:
Neu Kcb > 1: accu c nap u.
Neu Kcb < 1: accu b phong ien.
(: hieu suat nap.
2.3.3 Hien tng t phong ien
nhiet o cao se xay ra phan ng di day lam ch va oxt ch bien thanh
sunfat ch:
Pb + H2SO4 = PbSO4 + H2 (
2PbO2 + 2H2SO4 = 2PbSO4 + 2H2O + O2 (Dong ien cuc bo tren cac
tam ban cc do s hien dien cua cac ion kim loai, hoac do s chenh
lech nong o gia lp dung dch len tren va ben di accu, cung lam giam
dung lng accu. Do hien tng t phong ien, accu e lau khong s dung
cung se dan dan het ien.
2.4 Cac phng phap nap ien cho accu
Co hai phng phap nap ien cho accu:
2.4.1 Nap bang hieu ien the khong oi
Trong cach nap nay tat ca cac accu c mac song song vi nguon ien
nap va bao am ien the cua nguon nap (Ung) bang 2,3V 2,5V tren mot
accu n vi ieu kien Ung > Ua.
Cng o dong nap thay oi theo cong thc:
In = (Ung - Ea)/ (R
Hnh 2.9: Nap bang hieu ien the khong oiImax ( 1 ( 1,5 Qm.
Khi nap, Ea tang, I giam nhanh theo ac tuyen hyperbol.
Nhc iem cua phng phap nap nay la:
Dong ien nap ban au rat ln co the gay hong bnh accu.
Dong khi giam ve 0 th accu ch nap khoang 90%.
2.4.2 Phng phap dong khong oi
Theo cach nay dong ien nap c gi mot gia tr khong oi trong suot
thi gian nap bang cach thay oi gia tr ien tr cua bien tr R. Thong
thng ngi ta nap bang dong co cng o In = 0,1Qm. Gia tr ln nhat cua
bien tr R co the xac nh bi cong thc:
R = (Ung 2,6n)/ 0,5In
Hnh 2.10: S o nap accu vi dong khong oi
Theo phng phap nay tat ca cac accu c mac noi tiep nhau va ch can
am bao ieu kien tong so cac accu n trong mach nap khong vt qua tr
so Ung/2,7. Cac accu phai co dung lng nh nhau, neu khong, ta se
phai chon cng o dong ien nap theo accu co ien dung nho nhat va nh
vay accu co dung lng ln se phai nap lau hn.
n : so accu n mac noi tiep.
0,5 : he so d tr.
Ung : hieu ien the nguon nap.
2.4.3 Phng phap nap hai nac
Trong phng phap nay, au tien ngi ta nap accu vi cng o 0,1Im khi
accu bat au soi, giam xuong con 0,05Im. Phng phap nap 2 nac am bao
cho accu c nap no hn va khong b nong.
Hnh 2.11: Nap 2 nac
2.4.4 Phng phap nap hon hp
au tien, nap bang phng phap hieu ien the khong oi va sau o nap
bang phng phap dong khong oi. Co the nap nhanh oi vi bnh b can het
ien, nhng phai giam thi gian nap.
2.5 Chon va bo tr accu
e chon accu ta da vao cac ky hieu ghi tren vo bnh accu, tren cac
cau noi gia cac ngan hoac tren nhan hieu nh vo bnh, chu yeu la dung
lng nh mc cua accu, va cng o dong ln nhat ma accu co the phong ma
dong nay phu thuoc vao cong suat cua may khi ong.
Accu thng at trc au xe, gan may khi ong sao cho chieu dai day
noi t may khi ong en accu khong qua 1m. ieu nay am bao rang o sut
ap tren day dan khi khi ong la nho nhat. Ni at accu khong c qua
nong e tranh hong bnh do nhiet.
2.6 Cac loai accu khac
Ngoai accu ch axit con co cac loai accu kiem khac nh: Accu sat
niken (Fe Ni), accu caimi niken (Cd Ni ) va accu bac kem (Ag Zn).
Trong o hai loai au thong dung hn ca va a c dung e khi ong mot so
oto va may keo.
2.6.1 Accu sat niken
Ve cau tao, accu sat niken co the chia thanh hai loai: loai thoi
va loai khong thoi. oi vi accu loai thoi, moi ngan gom mi hai ban
cc dng va mi ba ban cc am. Cac ban cc cach ien vi nhau bang cac que
ebonit co ng knh 1,9 en 2,0 mm. Cac ban cung dau cung c han vao cac
vau cc va tao thanh cac phan khoi ban cc dng va cac phan khoi ban
cc am nh accu axit. Phan nho cao cua vau cc la cc cua moi accu n.
Tng khoi ban cc c at trong cac bnh sat co o dung dch ien phan gom
dung dch KOH vi ( = 1,20 1,25 g/cm3 va khoang 18 20 gam LiOH cho 1
lt dung dch. Cac ban cc c ngan cach vi vo bnh bang lp nha
vinhiplat.
Ban cc accu kiem loai thoi c che tao bang cach ghep hang loat
thoi chat tac dung lai vi nhau. e am bao o cng vng va tiep xuc tot,
ngi ta kep chat au thoi bang cach dap chat vi tai ban cc. Moi thoi
chat tac dung gom mot hop nho bang thep la cha chat tac dung. Chat
tac dung ban cc am la bot sat ac biet thuan khiet, con ban cc dng
la hon hp 75% NiO.OH va 25% bot than hoat tnh.
Moi ngan co nut va nap rieng. V sc ien ong cua moi accu n ch
bang 1,38V nen muon co bnh accu 12V, ngi ta phai ghep noi tiep 9
ngan accu n lai vi nhau, tao thanh 3 top accu. Nh vay trong lng cua
moi bnh accu kiem nang hn bnh accu axit kha nhieu, mac du cung the
hieu.
Loai accu khong phan thoi c che tao theo kieu ep bot kim loai co
cau truc xop mn. Chat tac dung c ep vao trong cac lo nho tren be
mat phan nhanh cua cac ban cc. Ket cau nh vay cho phep giam trong
lng cua bnh accu xuong 1,4 1,6 lan so vi loai thoi.
2.6.2 Accu Caimi - Niken
Loai accu nay ch khac loai accu sat - niken ve thanh phan hoa
hoc cua chat tac dung ban cc am, con cau tao va qua trnh hoa hoc
cua accu caimi - niken tng t nh accu sat - niken.
2.6.3 Accu Bac - Kem
ay la loai accu co he so hieu dung tren mot n v trong lng va
tren mot n v the tch ln hn hai loai tren, nhng v bac chiem ti 30%
trong lng chat tac dung nen viec s dung chung tren oto hien nay la
khong thc te. Cac cc cua accu nay la kem va oxit bac, con dung dch
ien phan, cung giong nh trong cac accu khac la KOH. Mot trong nhng
u iem quan trong cua accu loai nay la vi kch thc khong ln lam,
chung co the cho dong ln. Nhc iem cua no la tuoi tho ngan.
Bang 2.1 ien ap va nang lng rieng cua mot so loai accu
Loai accuien ap tren mot nganNang lng rieng
Accu ch axit
Sat Niken/ caimi
Niken kim loai hydrat
Natri lu huynh
Natri niken clorua
Lithium
Pin nhien lieu H2/O22.0V
1.22V
1.2V
2.0 2.5V
2.58V
3.5V
~30V30Wh/ Kg
45Wh/ Kg
50 80 Wh/ Kg
90 100 Wh/ Kg
90 100 Wh/ Kg
100 Wh/ Kg
500 Wh/ Kg
2.6.4 Pin nhien lieu (fuel cell)
Trong nhng nam gan ay xuat hien mot dang pin mi o la pin nhien
lieu. Loai pin nay ang c nghien cu va a bat au tm thay ng dung tren
mot so o to ien. Tren hnh 2.12, 2.13, 2.14 trnh bay mot so dang pin
nhien lieu thng gap. Nguyen ly cua pin nhien lieu da vao viec tach
electron cua nguyen t hydro e bien thanh dong ien bang cac phng
phap khac nhau.
Hnh 2.12: S o nguyen ly pin nhien lieu dang kiem
Hnh 2.13: S o pin nhien lieu dung kh ot
Hnh 2.14: S o pin nhien lieu dung hydro2e-
2e-
40
80
Q(Ah)
IP(A)
200
100
50
11,25% Qm
-18oC
Dong ien phong Ip = 3Qm
U(V)
t, h
4
6
8
10
5
4
3
2
1
+25oC
27,5% Qm
U,V
Inm
Inm
T=20oC
T=0oC
0
I,A
Ub
Ub
t,h
in
I(A)
0
tp+=
tn
ip
i
U=2,3V
Imax
In, U
t, h
A
I,A
_
+
0,1Im
0,05Im
t, h
E
_
+
V
A
A
V
iem cuoi qua trnh phong
Thoi nap
Khoang ngh
2,12V
(E
t(h)
14
10
8
6
4
2
0
t(h)
10
8
6
4
2
0
Eo
UP
Eo
Ip.Ra
(E
In.Ra
I(A),(,U(V)
2,0
1,5
2,5
1,0
B(2,70V)
1,96V
A(1,70V)
(
1,11
1,11
1,27
1,27
(
Ip
Un
0,5
In
IN=5,4A
Ip=5,4A
QN=IN.tN
2e-
2e-
Q=5,4.10=54
Ip
In
R
R
Eaq
Eaq
~
R
_1128926588.unknown
_1128940392.unknown
_1128940721.unknown
_1128941195.unknown
_1128926615.unknown
_1128926161.unknown
_1128926359.unknown
_1128925519.unknown
