Rather than use a conventional alternator to charge its 12V battery, the Honda Insight uses a DC/DC converter. This takes power from the 144V high voltage (HV) battery that powers the hybrid electric motor, and converts it to 14.3V for charging the 12V battery.
Despite being much more expensive and more complex than simply using an alternator, the designers of the Insight probably plumped for the DC/DC converter because it is likely to be more fuel-efficient than an alternator.
Well, the HV battery is partly maintained in charge by regenerative braking, which uses energy that would, in a conventional car, be wasted in heating the brakes. Thus, to put it simply, the 12V battery is charged by braking, rather than by the engine burning petrol.
The DC/DC converter is rated at 75 amps – which seems high when you consider the small size of car, three-cylinder engine, no electric seats, etc.
The space available under the bonnet for an alternator is very limited, and since alternator physical size goes up rapidly with increasing amps, I thought it might be a good idea to measure the actual current draw of the operating car. This way, as small an alternator as possible could be selected.
Getting a bit ahead of myself, I’d actually already started sizing and pricing small alternators, confident that something like a 40 amp Denso forklift alternator (they’re very small) would be sufficient if I didn’t want to run the sound system flat out… and have my 100W driving lights on… and be running the air con cabin fan at full speed.
In other words, I thought that a bit of simple driver decision-making could keep the current load pretty low. And, with the MoTeC dash going to be monitoring battery voltage, I’d be notified if anything was going astray anyway.
But when I made some measurements I was in a for a shock!
Normally, to measure max current you’d place a clamp-type current measuring adaptor around a battery lead and measure current draw with the engine not running, but with everything electrical switched on. Or, if you wanted to see what the alternator needed to produce, start with a fully charged battery and clamp the adaptor around the alternator lead and then again run all electrical loads.
In the Insight, I found the best place to measure current draw was on the output lead of the DC/DC converter.
Note: Making this measurement means opening the HV battery box and so potentially exposing yourself to a fatal electric shock. Do not even think of working on a hybrid car without a good understanding of the dangers – and resulting precautions you should take.
I placed the current clamp around the correct lead and started the car.
Immediately there was 16 amps current draw!
Turning on the lights brought the total to 29 amps, then turning up the cabin fan to ‘full’ brought the total to 35 amps.
Gosh, that’s already close to the ‘40 amp’ alternator I’d been considering!
The air conditioner (that automatically turns on the condensor fan) brought the total to 53 amps; the high beam headlights (including one of the two driving lights that was working) to 61 amps; and the windscreen wipers to 64 amps.
And that was without operating the electric power steering, having the brake lights on, or the radiator fan running!
Hmm, funny that - Honda got it right with the 75A rating of the DC/DC converter…
Selecting an alternator
About the only place to mount an alternator is behind the transverse engine, between it and the firewall. Placement in this location would allow the alternator to be driven by a longer version of the standard 5PK (5 rib) serpentine belt that drives the water pump and air con compressor.
Depending on the size of the alternator, such a placement would require either one or two idler pulleys – one idler to allow the belt to fit beneath the right-hand engine mount, and the other to give the crank pulley enough belt ‘wrap’ to allow it to drive effectively.
However, if a turbo were also to fit in the space between the firewall and the engine, the alternator size would need to be quite limited.
With the requirement now for an alternator of at least 75 amps, the size demand became difficult to meet. After a great deal of measuring and consulting of alternator catalogs (thanks Wayne at the Dalton Garage), I settled on a Denso 90 amp alternator, quoted in the catalog as fitted to the Lexus ES250 and just one model of the Toyota RAV4.
This alternator has the happy characteristic of being the same size as many 70-amp Denso alternators, but having a handy 20 amps greater output – presumably to run RAV4 driving lights and the electrics in the higher spec Lexus.
I sourced an aftermarket replacement alternator and paid AUD$305.
Fitting the alternator
As described in Relocating the alternator, mounting an alternator is a little harder than it first appears. This is the case because:
The implications for any alternator mounts that you may make are clear – they need to be really, really strong.
Making it even harder in the case of the Insight is that it’s nearly impossible to see where such strong brackets could locate on the engine block. Initially, I initially thought I’d have to take the engine out of the car but in the end I managed to build the bracket with the car placed high on jack-stands, the exhaust removed, and the right-hand engine mount out and the engine supported on a jack.
The Denso alternator is typical in design in that it has a wide lower ‘foot’ that forms the primary mount. The upper mount uses a single bolt designed to slide within a crescent shaped, slotted bracket to allow belt tension to be adjusted.
However, in the case of the Honda, a belt tensioner pulley already exists (the factory idler pulley is adjustable), so if a new belt of near the correct length could be obtained, the alternator could be mounted without the need for further belt adjustment. This is useful because no room near the alternator needs to be left for ‘belt adjustment’ clearance.
Designing an alternator mount was very, very difficult. This was primarily because of a lack of suitable bolts on the block to which the mount could be attached.
There was only one bolt directly below the proposed alternator mounting location – an 8mm stud helping to hold the sump on. Far to the left of this, there was a 10mm bolt that originally held the exhaust brace. To put this another way: if the lower bracket were to pick-up on two mounting bolts, it needed to be almost as long as the engine! (Just as well it’s only a 1 litre, 3 cylinder.)
The only strong mounting bolts close to the top alternator mount were those for the right-hand engine mount – and at least these were strong!
I therefore decided that the alternator mount be constructed so that the two wide-spaced lower engine bolts, and two of the upper engine mount mounts, were all utilised by the one large and continuous bracket. In other words, the single large alternator bracket would be held in place by two upper 8mm bolts (the engine mount), a lower 10mm bolt (the exhaust brace) and a lower 8mm bolt (the sump retaining bolt).
In addition, the two required idler pulleys could also be supported by this large mount.
Building the bracket
The bracket was an absolute bastard to make - it would certainly have been easier to fabricate had I taken the engine out of the car.
Problems in accurate alternator placement including allowing sufficient clearance between the bottom of the alternator and the torsional damper on the right-hand drive shaft, clearing the exhaust flange of the (still to be fitted) turbo, and of course lining up with the plane of the existing 5-rib pulleys.
The lower part of the bracket was made first. Two versions of this bracket were actually made – the first roughly from steel angle and square tube, tacked together. This bracket allowed the alternator to be trialled in various positions – it’s extremely hard to hold the alternator accurately in place if there’s nothing for it to sit on while it’s being lined-up and clearances measured. A new, second bracket then became the final iteration.
The upper part of the bracket required multiple templates (made from compressed fibreboard) as the bracket was complex in shape, needed to wrap around the alternator, clear all sorts of bumps on the block, and also provide mounting locations for the two idler pulleys.
The bracket was made from 8mm and 6mm steel plate, cut using 1mm cutting discs in an angle grinder. Steel spacers were turned-up in a lathe. The main bracket was MIG welded together while the steel spacers were brazed into place.
The right-hand lower mounting point – on the sump bolt – was made removable so that, if required, the sump could be taken off without the alternator having to come off first.
This pic shows the bracket after blasting but before being zinc undercoated and powdercoated.
Two non-grooved idlers were used – these bear on the back of the belt. One provides clearance between the belt and the right-hand engine mount, and the other improves the amount of belt wrap around the crankshaft pulley.
New Nuline replacement pulleys were selected. These are 65mm in diameter, 25mm wide and, very unusually, come with a two-piece steel bush that allows a 10mm through-bolt to be used. The pulleys are normally fitted to Daewoo, Chev and Holden products and are part number EP213. They are designed for 6-rib belts but of course will work fine on the back of a 5-rib belt.
The pulleys cost AUD$40 each.
The new drive belt is a 5PK1355 (this means 5 ribs, 1355mm long) that is available off the shelf. So that I could be certain that this belt was the right length, the belt was sourced prior to the second pulley being positioned. The procedure followed was:
This approach allows the belt to be fitted with the factory adjuster in its slackest position, with plenty of ‘belt tightening’ adjustment then still available.
The belt cost AUD$45.
The completed bracket weighs 2.1kg, the two idler pulleys (with bolts) just under 1kg total, and the alternator about 3kg.
The alternator mount was bead blasted, zinc undercoated and then powdercoated in wrinkle black.
Fitting the alternator took many, many hours. However, the result is a car that will be able to be driven without the presence of the DC/DC converter, giving much more flexibility in driveline configurations.
Next: the new airbox