Triumph Rat Motorcycle Forums banner
1 - 12 of 12 Posts

·
Registered
1977 Bonneville 750
Joined
·
4 Posts
Discussion Starter · #1 ·
I am a very new owner of a 1977 Bonneville 750. I’ve been struggling with kick starting it and it seems to be related to a weak battery. The photo below is the battery that was in the bike when I bought it a few weeks ago. I want to replace it - what’s the recommendation for an appropriate battery for this bike - I want a good one, not necessary a cheap one. Many thanks in advance to the help.
Motor vehicle Light Blue Automotive lighting Font
 

·
Registered
Joined
·
3,357 Posts
Hi,

This is the best battery I have tried, long life and negligible self discharge, 4 terminals for easy compatibility.

regards
Peg.

 

·
Super Moderator
Joined
·
11,110 Posts
Hi,
I am a very new owner of a 1977 Bonneville 750. I’ve been struggling with kick starting it and it seems to be related to a weak battery.
The advice so far is, at best, dubious:-

. All have assumed the battery is the problem when the battery doesn't start the bike, the ignition system does. That isn't to say a 'good' battery is completely unnecessary but your bike had points originally and I'm reliably informed by those that still cling to use them that they'll work with very little battery power.

. Otoh, points go out of adjustment fairly easily, one symptom of badly-adjusted points and timing is difficult starting ... a new battery won't fix badly-adjusted points and/or timing ...

So the first question of the owner of a 45-year-old bike should've been, "What ignition system does the bike have?"

The photo
is the battery that was in the bike when I bought it a few weeks ago. I want to replace it
I want a good one, not necessary a cheap one
Be careful what you wish for ... :cool:

The batteries listed above are jolly wonderful, fulfil your "not necessary a cheap one" wish; snag is they (like most "motorcycle" batteries) are made for electric-starting. Your bike wasn't fitted with an electric starter originally, has it had an aftermarket conversion?

recommendation for an appropriate battery for this bike
No electric-starter on your bike, the only other reason you might be happy with the weight and expense of a modern "motorcycle" battery like the Motobatt is if you expect to do a lot of low-rpm city riding and your bike still has the original alternator, which has a feeble output. However, afaict, no-one's asked those questions either ... :cool:

If you don't live in a city nor expect to do much low-rpm riding, there are wa-aa-ay cheaper and lighter batteries than've been suggested so far ...

Hth.

Regards,
 

·
Super Moderator
Joined
·
11,110 Posts
Hi Steve,
surprised Yuasa doesn't offer an AGM for the older bikes.
Not specifically for older bikes but, if they fit physically, they work.

Hth.

Regards,
 

·
Registered
1977 Bonneville 750
Joined
·
4 Posts
Discussion Starter · #9 ·
All good points (pun intended).
Regarding the battery, I’m somewhat relying on my smart charger which indicates during the analysis and conditioning phases that the battery is not dead but weak. Possibly the electrical system is draining it and that will be tackled next.
 

·
Super Moderator
Joined
·
11,110 Posts
Hi,
Regarding the battery, I’m somewhat relying on my smart charger which indicates during the analysis and conditioning phases that the battery is not dead but weak.
One thing to bear in mind is the battery in your photo. is 5 Amp-hours. Otoh, the battery Triumph fitted originally was 9 Ah and even the one I'd recommend absent electric-starter or much low-rpm riding is 7 Ah.

Second thing is, ime, 'smart' chargers can be as thick as two short planks ... :cool: My method for checking the battery itself:-

1. Trickle-charge it overnight.

2. Reconnect it to the bike, turn on and turn off the ignition switch - this will dissipate any misleading 'surface charge'.

3. Ignition switch off, connect a Voltmeter or multi-meter set to Volts across the battery - i.e. each meter lead end connected to a different battery terminal - note the meter reading. If the meter is well-calibrated, it should display 12.6V.

4. If possible, turn on just the ignition (no lighting), don't start the engine; at worst, the meter display shouldn't decrease by more than 0.1V.

5. If yes (still without starting the engine), then turn on all the lighting, including the headlamp; again, at worst, the meter display shouldn't decrease by more than another 0.1V (assuming no super-powerful headlamp and/or spotlamps?).

6. If meter still displaying no more than 0.2V drop from the reading at 3., leave everything turned on, observe the meter, note how long the display takes to reduce to 12V. Monitor the ignition coils temperature, neither (standard) should get more than warm.

Note 12.3V is a half-discharged battery, so the meter reading shouldn't fall 0.3V when just turning on ignition only and then lighting.

Possibly the electrical system is draining it and that will be tackled next.
Easy to check - trickle-charge battery overnight again, reconnect to bike, connect meter across it as before but don't turn on any switches. Note meter reading then recheck it periodically; if there's a drain, the meter will indicate it.

Other things to bear in mind
I never charge a battery on the bike. These old heaps, the battery's hardly difficult to remove for charging.

If you really must charge the battery on the bike, the main/only fuse must be removed or the (standard) Zener diode will just bleed off the charge.

Even with fuse removed, meter and/or charger leads connected to battery terminals, ensure there is absolutely no way the open seat can fall on leads connected to battery terminals (I unbolt the seat). Lucas made a fundamental error not placing the fuse in the 'ground' wire (as standard, not in just a single Red wire connected to battery +ve); if something metal (e.g. the seat pan) somehow connects to the battery -ve terminal itself (falls on a meter or charger connector?), it's an unprotected short-circuit that'll start to destroy the wiring harness literally almost immediately. :eek:

If you don't own a multi-meter, buy one; it's as essential for working on electrics as sockets and wrenches are for working on nuts and bolts. Pay a little extra for one (possibly from a local auto-electrician rather than those well-known auto-electricians Amazon and Ebay? :cool:) with:-

. A well-calibrated display - as I've posted above, 0.1V differences are significant when fault-finding.

. A cast-iron guarantee it has EMI (electro-magnetic interference) protection. Otherwise it'll be useless when the engine's running.

Hth.

Regards,
 

·
Super Moderator
Joined
·
11,110 Posts
Hi Steve,
135mm x 75mm x 139mm is the size Yuasa says it takes
Mmmm ... :cool:

The length is dictated by the height, which isn't brilliant because it puts the terminals and connections very close to the seat pan. (n)

Otoh, using a shorter battery not only moves terminals and connections away from the seat pan, it also allows the full battery carrier length to be utilised - Yuasa YTX9 (107 mm. high, 152 mm. long) and NP7 (<100 mm. high,151 mm. long) will fit.

Hth.

Regards,
 
1 - 12 of 12 Posts
Top