One of our customers recently had an incident were a generator starter battery exploded whilst it was on charge. Some electricians had moved the batteries so that they could work in that area and some time after they had put them down one gave a bang and sprayed them with battery acid. It wasn’t a pleasant experience for them. This kicked off a round of investigation to try and figure out what happened as well as what to do about it.
Given that these generators had been in service for close to 30 years and that this was the first incident of a battery exploding we looked for anything that had changed recently in the set up. As it turns out the type of batteries used had changed within the last couple of years. Previously flooded wet cell lead-acid batteries had been used, however when a new set of batteries was installed recently a set of automotive, ‘maintenance free’, flooded valve regulated lead acid batteries was used instead. The original batteries allowed the acid to vent without restriction which then required that they were periodically topped up to stop them going dry (about every 3 months). The maintenance free batteries on the other hand use a valve to restrict the hydrogen from being vented trying to force it to recombine within the battery. It has been a common trend with truck batteries to switch from flooded cell batteries to maintenance free starter batteries. This is largely because of the cost savings associated with not having to top them up regularly.
Taking to the web we started looking for other reported incidents involving maintenance free lead acid batteries. Worksafe Victoria had an excellent alert notice that explained the issues that we had seen.
The crux of the issue appears to be that the maintenance free starter batteries are not designed to be constantly charged by a float voltage charger. They are designed for running in trucks which have regular periods with the charger off (truck stopped), being slowly discharged (lights on but engine off) and having high discharge rates (starting the truck). However when used to start a generator there are very few times when the battery is not being held at its float voltage. This can lead to the acid gassing up and venting at a higher rate than if the battery is used in a truck. If this occurs there is now no way to top up the flooded cell which leaves a space within the battery for hydrogen to build up as well as leaving dry, closely spaced, electrode plates that can short to each other and provide an ignition spark.
With a likely cause in hand we moved on to working out what to do to remedy the situation. In this case our customer requested that the batteries be replaced with a set of AGM batteries. These are the same type of batteries that we regularly use in UPS applications. They are designed to handle constant float charges. There was however a question mark over how they would handle the peak power draw required to start a large generator.
The batteries we were replacing were ACDelco SN200. They are rated as having a cold cranking current (CCA) of 1200A and a 20hr discharge capacity of 200AHr. The advice from our supplier was to use Vision 6FM200PX batteries, which have a matching 20hr discharge capacity of 200AHr. However there is no mention in the data sheet of the CCA rating. In order to assess how the new batteries perform when starting a diesel engine we took recordings of the battery voltage and current during a start using both the new 6FM200PX and the original SN200 batteries. The charts showing the voltage and current of the respective batteries during the two tests are shown below.
In these two specific examples we can see that the generator was started 2 seconds faster by the new batteries compared to the original batteries. This is likely due to the new batteries being able to provide more power to the starter motors, getting them up to speed quicker, allowing the generator to start. The power and energy provided by each battery is shown below.
By providing more power to the starter motors initially the generator was able to be started quicker by the 6FM200PX batteries leading to significantly less energy being drawn out of the new batteries. This should lead to less wear and tear on the new batteries during generator starts.
This is based on two specific starts and the performance of the new batteries during future starts may change. The monitoring equipment has been left in place and is set up to record any future starts on Generator 4. We will review the data from these events during future preventative maintenance services.
So it looks like the new AGM batteries are going to be up to the task of starting the generators. They will also handle the float charge well too. Hopefully we won’t have any more exploding batteries going forward.
Feature image includes artwork created by anbileru adaleru from the Noun Project