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The Challenge

PPT was approached by one of the U.K.’s leading ambulance builders to investigate premature failure of auxiliary battery banks in their frontline ambulances. With our experience in battery management in auxiliary power systems, PPT was contracted to go on site to an ambulance station and look at the setup of the existing system, in particular the complex contractor arrangement used extensively in most front-line ambulances. This is fitted to ensure priority loads always get power, enabling the ambulance to start, should it be on scene for a long period and have heavily discharged comms, auxiliary and starter batteries.

Our Solution

We commenced an investigation by looking very closely at the failure mechanism of the AGM batteries. It became immediately apparent that the batteries were seeing overvoltage during charging from the alternator. Further investigation of data logged by the NHS trust for this vehicle, showed that charging cycles during driving were short and on many days depth of discharge (DOD) was seen to be high.

Typically, auxiliary power systems and frontline ambulances haven’t changed dramatically. They incorporate a complex split charging system with contactors that in the past was a low cost and fast way to recharge the auxiliary batteries. With the change from old-style fixed voltage output alternators systems to variable voltage alternators to meet new EU 6 regulations, no consideration had been given to the split charging system and how that would work with the new alternators. It could clearly be seen that the higher voltages from the alternator were damaging the AGM batteries causing them to heat up and eventually bloat and fail. Charging the AGM batteries from smart alternator and contactor split charge system only meant that in-vehicle charging was ineffective (and this could be seen in the discharge and recharge cycles from the data) plus the batteries had no protection from the higher output voltages from the alternator.

Reviewing the wiring diagram and schematic with our customer we removed part of the old-style split charge system and replaced it with high quality fanless DC-DC charger that is compatible with variable voltage alternators. The DC-DC charger would accept a wide range input from the alternator whilst providing a three stage charging profile that would charge the AGM batteries quickly and in a gentle manner. This solution would therefore ensure effective in-vehicle charging whilst protecting the batteries and extending their life. Since most ambulances utilise multiple battery banks serving different loads and communications, a DC-DC charger was required for each bank of batteries.

PPT recommended the Redarc BCDC1240D which is highly reliable in professional applications and is well tested by us in many vehicle types.

Since the change over to the solution, AGM batteries have stopped failing, watt hour capacity has improved, and the ambulance fleet has seen increased performance in their day-to-day power availability.


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